Hi! I am the mom of Adam who will be turning 4 on the 19th of October. He was potty trained last March. What we finally ended up doing is when we watched TV, made him go potty after every show was over (we watch PBS, so there were no commercials). It helped, because he was pretty scheduled to go every 1/2 hour or so. We tried prizes, stickers, candy, and they didn't work at first.
He didn't really have an accident, pee-wise, but when he pooped in his underpants, I made him clean them up in the toilet and then help with the laundry. WHen he was doing it too much, I finally told him that if he did it again, I was going to put him in a diaper like a baby. Well, big boys don't wear diapers, so that stopped him. I never yelled at him, though, it was just matter of fact. (Oh, and I would put the diaper on backwards, so he couldn't just rip it off-it took a little bit of work to take it off).
He was potty trained in two weeks that way. He *still* wears a pull up to bed at night, but I can handle that. We're buying a package of pull-ups every 3 months, as opposed to every week and a half.
HTH!