Others have answered, but I wanted to add that you could build a good, strong relationship with the God/dess who granted you the favor. They appreciate that too.
If one of the ancient Gods (Goddess) grants you a favor, how do you repay their kindness? (yeah, it's Satanism 101, but I'm new to this form of it, so please don't be sarcastic with the answers; this is a VERY serious matter!)