When asking for help, appeal to people’s self-interest, never to their mercy or gratitude: if you need to turn to an ally for help, do not bother to remind him of your past assistance and good deeds. He will find a way to ignore you. Instead, uncover something in your request, or in your alliance with him, that will benefit him, and emphasize it out of all proportion. He will respond enthusiastically and he sees something to be gained for himself.
This reminds me of my time is spent in college microbiology, where I studied the concept of symbiotic mutualism. Two organisms benefit by collectively working together. To me this concept is very natural.