I don't know how true it is, but I've heard that its better to try to stick to a single flavor rather than give them a variety - that way they don't know that there is a better one that the one they are getting.

I did that with my gang. I tried different kinds until I found the one that all of them liked. (for my gang, its the chicken/tuna combo). Now I don't get any other flavor, and so far, no one is rejecting it.

I do also just give them the canned food as a snack - their dry food is their main meal.