We've had the same problems with my 3 1/2 y/o son. We resorted to no underpants or pants on during any time in the house. Every hour, I put him on the potty, whether he says he "needs to" or not. Generally, there'll be something there! And we go before we leave the house to go anywhere, and when we arrive (even if it's a store), we'll go again.
I think that keeping him "on task" about potty training (i.e. making him go every hour) is beginning to train him again. He has been accident free for 3 days now. We reward him ONCE a day for poops on the potty - he gets one gumball (it used to be M&M's but that got out of control). He looks forward to his gumball daily. Will be weaning him off that soon.
Patience is the biggest thing...for the moms. I have very little, and I learned the hard way that if I don't have patience, he has more accidents.