I had a lot of trouble choosing between TO and FOR myself, and I still do sometimes. The first thing you're supposed to understand is that there are no rules. What helped me most was the meaning of the to words:
- TO: in the direction of
- FOR: in favor of
- You send something TO someone. (I've sent the letter TO you.) or
- You send someone something. (I've sent you the letter.)
(I would prefer the second option.)
Take a look at the following phrase:
- I've sent the letter FOR your happiness.
I guess you won't have problems to understand the difference between the two cases with the meanings of the prepositions in hand.
Hope I could be of some help.