Hi S. - I just came through this battle with my son 3 weeks ago. He turned 4 on Jan 7th.
I started trying with him off and on since he was 2 1/2. We told him that when he was a big boy who didn't wear diapers or pull-ups and used the potty that we would buy him a big boy bike. We read potty books to him, watched all the potty DVD's. I probably tried once every 3 months or so - it never clicked with him. Stubborn, stubborn, stubborn that boy is. He would argue, claim that he was never going to grow up, he wanted to be a baby forever.
What finally worked for us was taking away the pull-ups (except at night) - he wore his underpants only during the day. I did that for a week, and he had quite a few accidents. When there was an accident, I told him that he had to 'help' clean it up. He did not like putting the poop from his underpants in the potty and a couple of time it had fallen on the floor and he had to pick it up - he didn't like that either, LOL! I had him put the soiled underpants in the laundry and then made him got upstairs to his dresser to get a new pair. I even made him put on the new underpants himself, which was comical to watch.
I told him that he couldn't go to pre-school until he was potty trained, he loves his pre-school T/Th mornings - he missed one weeks worth of school before I felt confident that he would not have an accident.
After the week, I then went to the dollar store and bought about 15 matchbox cars and put them in a lidded see thru container on the top of my fridge. I told him that if he didn't have any accidents and kept his underpants clean until Daddy got home, he would get to pick one of the cars. This was HUGE for him, he wanted those cars bad. He only had two minor drippy kind of accidents and when he had about a weeks worth of cars left, I told him that when the cars were gone, we would take him to the store to get that Big Boy Bike!
Everyday I would call my Mom and my MIL and gossip to them about how great he was doing, how he got another car last night when Daddy came home, how when all the cars were gone, we would get him a Big Boy Bike... I would put them on the speaker phone and they would tell him how excited they were about him being a big boy etc.
Last Monday he told us he didn't need the pull-ups at night anymore, and he hasn't had any bed accidents - yay!
Last night we went to the store and bought that bike!
For me the key was a combination of no pull-ups, rewards, no pre-school, gossiping, and the big boy bike.
Although, I am not sure if it was all of the above, or if he was just finally ready himself?!?!?
I hope this helps you.
W.