I'm not gonna be that big of a help! because I only have 1 child and he is 16 months, so no potty training yet! But I do work w/ kids in the potty training age range.
I've noticed some moms getting their kids a candy bar/small toy/trip to McDonalds and sometimes it seems to work.
In the preschool class I teach, we give stamps to the kids 2 times during the class(on their hands, like winnie the pooh, rugrats,etc.). Once 1/2 way through class, and again at the end. It is an incentive for them to do things like share/clean up/no hitting, etc. And it works!
And one mom told me she bought stamps and is giving it to her son for potty training and it seems to be working pretty well!
And another idea: make a little chart for the day and everytime he does pee pee or poopie in the potty , he gets a sticker and at the end of the day(or late afternoon) count the stickers together and (no matter how many there are), he gets a lolipop/candy/cookie,etc. You can even have special stickers for poopie, so he knows that the big sticker or the "special" sticker means he did poopie and maybe he can get an extra special treat or a bigger treat.
And there is a video tape out there for this, it's songs about potty training Potty time or something....I saw it at babys r us one time.
Hope I helped.
GOOD LUCK!!