Bhagavad Gita Verse 28, Chapter 4

dravyayajnyaastapoyajnya yogayagnyaastathaapare |
svaadhyaayagnyaanayagnyaashcha yatayaha sanshitavrataahaa || 28 ||
 
Others offer sacrifice of materials, austerity and yoga, and other seekers with a resolute will offer the sacrifice of knowledge through study of scriptures.
 
dravya-yajnyaaha : material-sacrifice
tapo-yajnyaaha : austerity-sacrifice
yoga-yagnyaaha : yoga-sacrifice
tathaa : and
apare : others
svaadhyaaya : study of scriptures
jnyaana-yagnyaaha : knowledge-sacrifice
cha : and
yatayaha : seekers
sanshita-vrataahaa : with resolute will
 
Shri Krishna gives us a choice of four more yajnyas in this shloka.
 
First, he talks about the sacrifice of wealth, or as it is more commonly known - charity. As we saw in the second chapter, lobha or the tendency to hoard can destabilize our mind, and strengthen the ego. Therefore, giving away wealth or even our time to a higher ideal checks this tendency to hoard. But charity has to be done with the attitude that I am giving away what was not mine to begin with. If one donates with a view to gain publicity and so on, that is a selfish or rajasic type of charity.
 
Secondly, Shri Krishna mentions austerity or tapas. In this type of yajyna, the urge of the sense organs to go out into the world is checked, so that the ego is weakened. There are three avenues for conducting tapas: the body, senses and mind. In physical tapas, we use the energy of our body to do seva or service the world. In sense-related tapas, we keep a strong leash on our senses and organs. For example, we can practice austerity on speech by always speaking truth, saying what's beneficial to someone, and creating disturbance in anybody's mind. In mental tapas, we control our mind by not giving attention to negative thoughts and emotions, and not letting others trigger such emotions in us.
 
Third, Shri Krishna gives us the option of practicing a detailed regimen of spiritual practice or yoga. It could be bhakti yoga (which we will see later), karma yoga, raaja yoga of Patanjali and so on.
 
Finally, we can practice study of the scriptures, which is also known as jnyaana yajnya. A daily reading of the Gita, Ramayana or any other such spiritual text with utmost attention, concentration, understanding and discipline is also a yajnya. Here also, the ego becomes weak because the intellect gains a firmer and stronger position in relation to the ego, strengthened by daily exposure to the scriptures.
 
The common thread of all the yajnyas mentioned is that of weakening the hold of the ego, which is nothing but weakening of the notion of "I-ness" and "mine-ness".