M.M.
I had the same problem with my son. I finally decided that we would "run out" of diapers. We started a count down at 10. Everytime we changed his diaper we would count how many more were left (and get excited about it!). We didn't bring it up at any other time. When we ran out he held it as long as he possibly could but finally (with about 30 minutes of fussing) gave in and went on the potty. When he finished he said "that wasn't so bad!"
For him, it was important that we didn't "hound" him all the time about going. He would tell us when he had to. We had to kind of learn to back off and trust him.