Bhagavad Gita Verse 51, Chapter 18

buddhayaa vishuddhayaa yukto dhrityaatmaanam niyamya cha |
shabdaadeenvishayaamstyaktvaa raagadveshau vyudasya cha || 51 ||
 
Endowed with a pure intellect, and regulating oneself with fortitude, giving up sense objects beginning with sound, and eliminating like and dislike.
 
buddhayaa : with intellect
vishuddhayaa : pure
yuktaha : endowed
dhrityaa : with fortitude
aatmaanam : oneself
niyamya : regulating
cha : and
shabdaadeen : beginning with sound
vishayaan : sense objects
tyaktvaa : give up
raagadveshau : like and dislike
vyudasya : eliminating
cha : and
 
In this and the next two shlokas, Shri Krishna illustrates the lifestyle the monk who has entered the stage of sanyaasa or renunciation. The urge to perform a lot of physical actions is low for such a person, otherwise it is impossible for him to even enter a monastic life. Now, the focus shifts towards regulating the mind. Since the monk has already eliminated his strongest material desires, he has to focus on the ones that are weaker, with a view of completely eradicating them as well.
 
Such a sanyaasi has a buddhi or intellect which is vishuddha, extremely pure, free from any doubts and misconceptions about the nature of the material world. There is not even a trace of doubt about the existence of joy in inert objects. His dhriti or fortitude is saattvic, as we have seen earlier in the section on fortitude. He has complete mastery over his mind. He is able regulate his mind by his intellect, and prevent it from wandering into materialistic thoughts. Even if that does happen, his vision prevents those thoughts from causing desire. How is he able to do that?
 
This world is nothing but names and forms, and forms are comprised of five elements - space, air, fire, water and earth. Shabda adeen in this verse refers to these five elements. The sanyaasi is able to look at any object or person, see through the name and form, and understand that it is nothing but the five elements combined together. Realizing that any combination of these elements is nothing but Prakriti or matter, he is able to eliminate any sense of raaga or dvesha, like or dislike, towards that object or person.

Bhagavad Gita Verse 9, Chapter 15

shrotram chakshuhu sparshanam cha rasanam ghraanameva cha |
adhishthaaya manashchaayam vishayaanupasevate || 9 ||
 
The ear, eye and touch, taste and smell, taking support of these and the mind, it enjoys the sense objects.
 
shrotram : ear
chakshuhu : eye
sparshanam : touch
cha : and
rasanam : taste
ghraanam : smell
eva : only
cha : and
adhishthaaya : taking support
manaha : mind
cha : and
ayam : this
vishayaan : sense objects
upasevate : enjoys
 
Shri Krishna continues explaining how the jeeva, the individual soul, takes up a human body to carry out its transactions with Prakriti. We can use the wonderful Vedantic text "Drig Drishya Viveka", or "Discrimination of the seer and the seen", to examine this process in a little more detail. It explains, with almost mathematical precision, how the eternal essence begins to think of itself as a finite human entity. There are two main players in this process. One is the eternal essence, which is of the nature of infinite awareness and existence, explained in great detail in the second chapter. Let us call it "Om" here. The second player is avidya or ignorance, which exists in the realm of the illusory Maaya.
 
When Om and ignorance come into contact with each other, a fragment of Om is reflected in ignorance, just like a fragment of the sun as though appears in the water of a pot. As we saw earlier, this fragment is called the jeeva. Ignorance creates an apparent limitation, also known an upaadhi, which imposes a sense of incompleteness or finitude upon the jeeva. The jeeva seeks the help of the upaadhi to remove its sense of completeness. The upaadhi can exist in one of many modifications or states. Let us examine these modifications. The sense organs are modifications of this upaadhi that can contact different aspects of Prakriti. The mind is a modification of this upaadhi that can take input from the sense organs, compare that input with its memory, and present a complete picture of what was contacted. The ego is another modification that thinks of itself as the "I", just like a low level manager thinks of himself as the owner of the factory in the absence of the real owner.
 
Now let us examine the content of this shloka. After the end of its journey in one body, the jeeva travels with the mind and senses, the upaadhi in other words, in search of another physical body. When the ego aspect of the upaadhi associates itself with a tiny physical body inside a womb, based on the desires its wants to exhaust, it develops the notion "I am this body". It now begins to use the body to transact with the world. The human nose, for instance, is an inert object. But due to the association of the ego with the body, we think that the physical human nose is doing the smelling. The ego then uses the mind to generate the notion "I know that this smell is that of a rose". This finally leads the ego to generate the notion "This smell it pleasant, it makes me happy".
 
In this way, the jeeva gives up its power of reality, knowledge and happiness to the ego. The ego becomes the doer and the experiencer of the world. It has to constantly transact with the world in order to chase sense objects for happiness. It forgets the fact that it was happy to begin with. It gives reality to the domain of Prakriti, the three gunas, the visible and so called tangible world. But we should not forget that behind all this is the illusory mix up of Om and ignorance, the mix up of awareness and inertness. The Raamacharitamanas describes this mix up as "jada chetan ki granthi" or the knot between awareness and inertness.

Bhagavad Gita Verse 64-65, Chapter 2

raagadveshaviyuktaistu vishayaanindrayaishcharan |
aatmavashyairvidheyaatmaa prasaadamadhigachati || 64 ||
 
prasaade sarvaduhkhaanaam haanirasyopajaayate |
prasannachetaso hyaashu buddhih paryavatishthate || 65 ||
 
But, the one whose mind and senses are under control, is devoid of attraction or revulsion. He moves around objects and gains the state of tranquility.
 
Having gained tranquility, all of his sorrows are destroyed. His mind is joyful and his intellect soon becomes steady.
 
raagadvesha-viyuktaih : devoid of attraction or revulsion
tu : but
vishayaan : objects
indrayaih : through senses
charan : moves around
aatmavashyaih : subdued
vidheyaatmaa : one with a controlled mind
prasaadam : tranquility
adhigachati : gains
 
prasaade : gaining tranquility
sarva-duhkhaanaam : all sorrows
haanih : destroy
asya : of this individual
upajaayate : become
prasanna-chetasah : joyful mind
hi : surely
aashu : soon
buddhih : intellect
pari-avatishthate : steady
 
In the last shloka, Shri Krishna explained the "ladder of fall", or how constant thinking about material objects leads to a fall from equanimity. In this set of shlokas, he explains the exact opposite scenario, where bringing the senses and the mind under control brings us to a state of happiness. Here's the sequence of events:
 
Bring senses and mind under control -> one becomes devoid of attraction and revulsion -> he can experience the material world without any problem -> his mind becomes tranquil -> his intellect becomes steady -> he has no more sorrow -> he attains the state of happiness.
 
So, if one continues to pursue one's svadharma, and stay devoted to a higher ideal, one gets to a stage of equanimity. We have learned this in earlier shlokas. But then, what next? This set of shlokas tells us that performance of svadharma has a purifying effect - it is like a flame that burns away our vasanaas. As the vasanaas burn away, our minds remain situated in equanimity - and that's when our sorrows diminish.
 
We are always looking at quick fixes to be happy - new job, new friends, read a new book, move to a new place etc. But what comes across in these shlokas is that a long-term state of happiness cannot be found in a quick fix solution. All we can do is follow our svadharma, fix a higher goal, and keep at it.
 
The eight shlokas including this one comprise the answer to the fourth of Arjuna's four questions, "how does a person of steady wisdom walk", in other words, how does such a person control his mind?

Bhagavad Gita Verse 62-63, Chapter 2

dhyayato vishayaanpumsaha sangasteshoopajaayate |
sangaatsanjaayate kaamaha kaamaatkrodhobhijaayate || 62 ||
krodhaadbhavati sammohah sammohaatsmritivibhramaha |
smritibhramshaad buddhinaasho buddhinaashaatpranashyati || 63 ||
 
When a man constantly thinks about objects, attachment for those objects arises. From attachment is born desire, and from desire is born anger. From anger comes delusion, from delusion comes loss of memory, from loss of memory comes destruction of intellect, and once the intellect is destroyed, he perishes.
 
dhyayataha : thinking
vishayaan : objects
pumsaha : individual
sangah : attachment
teshu : to those
upajaayate : is born
sangaat : from attachment
sanjaayate : is born
kaamaha : desire
kaamaat : from desire
krodhah : anger
abhijaayate : is born
 
krodhaat : when anger arises
bhavati : happens
sammohah : delusion
sammohaat : from delusion
smritivibhramaha : loss of memory
smritibhramshaad : from loss of memory
buddhinaasho : destruction of intellect
buddhinaashaat : from destruction of intellect
pranashyati : he perishes
 
Earlier, Shri Krishna touched upon the topic of continually thinking about material objects when we do not contact them physically. In this shloka, he goes into great detail as to why it is to be avoided. These two shlokas are sometimes referred to as the "ladder of fall". They illustrate how one simple thought can lead to the downfall of an individual.
 
Here is the entire sequence of events as mentioned in the 2 shlokas:
 
Constant thinking of material objects -> attachment -> desire -> anger -> delusion -> loss of memory -> destruction of intellect -> individual perishes.
 
Let's first examine the sequence of events from constant thinking all the way upto desire using the Ipod example from earlier :
A person has thoughts about ipod on sale -> gets attached to that ipod deal -> desire to buy ipod on sale is created.
 
Now, at this point, he has purchased the Ipod and is extremely attached to it. Even without going forward in the sequence, we can see that the material desire to procure an Ipod has taken the person away from performing his svadharma. His equanimity has already been disturbed. He is dragged back into the material world, and has taken one step backwards from moksha or freedom.
 
Let's move forward in the sequence:
Ipod breaks down -> he is angry that it has broken down -> he completely loses his equanimity -> takes anger out on his wife -> family environment is agitated.
 
Here, not only has he completely lost his equanimity, but has also caused pain to other members of his family. So the message here is that constant pondering and thinking about objects eventually leads to moving away from equanimity, hence it is to be avoided, or at least minimized to the extent possible.
 
An interesting point seen here is around anger. Per the shlokas, anger is caused when one's desire gets obstructed. Also, attachment to a concept such as one's job title or position causes continual waves of thought, anger and delusion, in other words, stress.  Therefore, these shlokas provide an ancient but relevant analysis of anger and stress.