I would like to add one point. People have this idea that they will daven for someone else who needs the same thing and thereby get it themselves. I believe this is a misinterpretation of the chazal.
If despite one’s own needs, one is able to care even more about the problems of the other, and so davens for him, this shows a deep concern and high level of chesed and rachamim- the purpose of creation. It easily follows that such a person deserves an extra level of rachamim from Hashem.
If however, a person davens for someone else BECAUSE he thinks it will help himself, this only reflects on his concern for himself. He is merely using the other as a tool to fulfill his own needs. (Perhaps he is even happy to find someone with the same problem so that he can daven for him and be answered first.) The oft quoted statement of chazal does not really apply to this person.