We still have this problem off and on with our five year old, too. You're not alone.
My best tip is to scrap the discipline. We've tried it all, from the positive reinforcement to punishment. Probably the same as you've tried. Nothing worked, until we just dropped it altogether. If he has an accident, we're nonchalant about it. "OK, put your clothes in the washer and get new ones." That's it.
We've also been playing up kindergarten and how five year olds there won't have accidents and that teachers there don't clean up after kids who have accidents. Just this morning I mentioned I forgot his clothes (swim clothes) in the car at preschool and he said, "Don't worry, mom. I won't have an accident. I'm five now." So, I think that's getting through to him.
EDITED TO ADD: Also, be sure to take him to the potty (don't ask) before you go to any activities. I found that helped reduce the frequency of problems.
I know in rare cases, there's a medical reason but our pediatrician saw nothing wrong with our son, except his total lack of interest in the potty. You are not alone! I know it feels that way, so know that your situation isn't unique.
Best of luck!
M.