I wrote this particular for my friend who is trying to potty train her son, but Blogger gives me trouble and it wasn't letting me actually post the comment...so, I thought I'd publish it for all to see if they so choose.
I don't know what you have done, so please don't be insulted if you've already tried this. I haven't done it myself but have watched several and assisted in a few more who have done it successfully.
You start out by taking them to the store and allowing them to pick out their own big (boy) underwear and a few treats. You can pick stickers, candy, whatever you are comfortable with AND they want. Then, when you get home you put all the fun stuff in a bowl and tell the child that when they go they will get to pick any treat they want from the bowl. Then, you have them put on their new undies. Then, you play a game with them...go to every room in the house and "practice" running to the toilet when they have to go. Make it fun...do a race type thing, and just go room to room, practicing how to get there (and, you can practice pulling down the pants, sitting on the potty, and washing hands if you want). Then, remind them about the prizes (put it out of reach but where they can see it, if you can). From then on you ask them about wanting to go and remind them that if they go on the potty they get the prize..do this every half hour or so, or, when you start seeing the signs. Other than that, stay in the background with it, letting them play and such as normal, don't bug too much. Approach it from the angle that they will get the prize when they go in the potty. You will probably have a mess or two in the process. Most people I've seen have done it with a diaper/pull ups at nap time/overnight at first, then after a few days they see the child is naturally holding it and its time to switch to underwear then too. I'm sure there are some exceptions to this method, and although I'm not one to use bribery as my main parenting technique from all accounts this works pretty good because it is a game to them and a reward system.