I'm told that this is a common problem among boys. THey prefer to play rather than to stay clean. I know my three year old deals with this problem. Here is what we did with him. It helps, but hasn't totally gotten rid of the problem.
Every time he has an accident I take away whatever he was playing with, watching, or doing that was more interesting than the potty. I'd take away his t.v. Remove it from his room, just to get the point across. Set a time frame for when he'll get it back. Then if he does it again, lengthen the "Toy timeout" penalty. Ours was one day with my son, but you may need longer with an older kid.
Good luck!