my son didn't actually master the potty until about 3 1/2. A friend of mines boy was almost 4.
they thing that worked the best for us was constant, regular trips to the bathroom. At one point we would give him a glass of water to drink while sitting on the potty, to flush fluids through him so he could really become aware of needng to potty before he went.
sometimes his protests of sitting on the potty wasn't a matter of not wanting to do it, it was wanting to make the decision himself. so we would say something like, "Sam, can you go sit on the potty now." or "Let's go sit on the potty."
When he did have an accident we had him clean himself up. You can stand him in the tub and hand him wipes and hold the trash can.
But when it comes right down to it, he'll do it when he's good and ready. Our son just one day, said, "I need to go potty" and went up all by himself.