my little charge did this around the time of her 3rd birthday.... having been quite reliably potty trained since 23 months
basically we did the following:
In the morning she chose her pants.... has loads of pink / sparkly / charachter pants... and had some influence on her clothes
If she had an accident she was given white (or the most plain and boring pants that we could find) and her least favourite clothes out of a rather extensiive wardrobe for a 3 year old 
Basically if she said she needed the loo we put EVERYTHING that we were doing on hold.... we paused the telly (sky+) IF it was on (hardly ever watch TV but anyway)..we would 'pause' any games we might have been playing / stop building lego (or whatever) and wait for her to come back before we continued... loads of praise etc...
(I would go with her and say well done / good girl etc!)
IF she had an accident the following happened...
whatever she was doing she had to stop immediatly (no 1 more lego brick etc)....
she would have to walk herself to the toilet (NOT be carried)
sit on the loo and try to do some "more"
take her wet clothes off herself
Carry her wet clothes herself and put them in the laundry basket
Walk upstairs herself...
Be given her next set of clothes (she had no choice, unlike in the morning)
put on her clean clothes by herself (she would get a bit of help with tricky things she really couldnt manage, but she COULD manage to dress her self in most clothes IF she persevered... so she had to
then go and wash her hands by herself
ALL while all other play and activities continued as normal!
During which time there was as little communication as possible.... Ie Sit on the toilet.... do a wee, put these in the wash basket etc.... no general chit chat conversation at all
IE it took a LOT longer if she had an accident.... she had to sit on the loo anyway and wash her hands regardless AND would miss out
This worked pretty much perfectly after a week, and she has been almost bone dry ever since!