Fourth Yama of Ashtanga Yoga
Asteya is non-stealing. This one is pretty self-explanatory. However, it is good to bear in mind that there are many subtle ways to appropriate what does not belong to us. As for the other yamas, much self-analysis will be necessary to catch the subtle lower tendencies of our mind.
Desire or want is the root cause for stealing. Swami Sivananda
Asteya also includes the concept that you should try to be content with what comes to you by honest means. If you are always dwelling on things other people have and you don't have, eventually the thought of taking something that doesn't belong to you becomes more acceptable to the mind, which can in turn lead to actual theft. When you eventually convince yourself that someone else "has so much and won't miss this if I take it", you are giving yourself permission to steal. This method can reduce feelings of guilt in the short run, but in the long run is still a violation of this yama.
Most people aren't aware of it, but the idea of hoarding is another aspect to asteya. The idea is that you are keeping more than what you need for yourself instead of sharing or giving things away that you no longer need. Hoarding applies to many things such as food (eating too much), money, and possessions. Of course one should always keep what is reasonable and necessary to provide for one's self and family members, but a thoughtful analysis should be made as to what is actually necessary to keep, and what one is keeping because of various attachments.
If you are established in non-stealing, all wealth will come to you. Swami Sivananda
In yoga we are taught that when we no longer desire something it will come to us by itself. This is also the case for asteya. Since the root cause of taking what doesn't belong to you is desire, when you give up desire for things, all sorts of wealth will come to you by itself. To master this yama we should try to curb our desires little by little through the regular practice of yoga, and eventually the mind and our actions will come more under our control.