you know what....you have reached a stage my son got to, when i finally just said, "you know what, you KNOW how to do it, so it's time to be a big kid." i would try putting her in panties, and letting her know, that she is now expected not to have accidents. only you know how deliberate are her "accidents" - a time out might even be in order, if you feel she is really doing it deliberately. there comes a time when "convincing her" is over. she KNOWS. therefore she needs to do it. it's the first time as a little person that she is expected to do something, it's time to teach her what it's like to have expectations and to be responsible for something. this is her job as a big kid. no more being a baby and going in our pants. good luck!
(if you feel she still needs some "convincing", i.e. that she's not deliberately going in her pants, just needs a bit more of a reward, maybe try doing an m&m or a tic tac when she goes BY HERSELF. meaning not when you tell her to go, etc. but regardless of how you go, obviously the current method isn't working. as moms we all try different tactics. it's a matter of knowing when the current one has outlived its usefulness!)