You just described my son at that age. Nothing worked... or so we thought. We stopped - completely (back in diapers, never mentioning it, etc) - for a few weeks. When we started up again, we had him earn poker chips. This was right around his 3rd birthday so he was old enough to understand and so your daughter should, too. He earned 1 poker chip for going pee, 2 for poop and lost 2 for accidents. Didn't matter if we told him or he went in on his own. Each day started over at zero. When he earned 5 of them, he got to play the PS2 for 30 minutes (he was obsessed with the Star Wars Lego game). After a week he was consistently earning it everyday so we upped it... he only got poker chips when >he< told us he had to go (or just went in by himself), no reminders and he had to earn 10 a day for the 30 minutes of video games. This was the only computer or PS2 time he got so it worked. He learned simple math really quick, too... by the 2nd week he'd look at the ziplock with the chips and figure out how many more he needed to earn his reward. :-)
Figuring out what would work to motivate him took awhile. None of the typical stuff like praise, stickers, candy, big toy at end of week, etc worked with him for more than a day or two. It might take awhile to figure it out for your daughter. Is it TV? video games? playing with Barbies? going to a playground? riding a bike? soccer? dance?
Is she around older kids at all? That is helping my 2 year old right now... she plays with her older brother and sister and their neighborhood friends (mostly 4-8 year olds) and she wants to be a big kid like them. She knows they aren't in diapers... peer pressure can be a good thing. :-)
Good luck! She'll get it before you know it.