Not sure I'll be much help here, but this is what we're doing.

The potty has been around the house for ages and we talk about it a bit. There have been a few accidents during the last few months when she has wanted to wear undies, but we have had some success too. We've generally left it up to her, sometimes she just decides that she wants to wear undies for a while.

On the weekend, we sort of pushed the undies a bit more - but got a bit of resistance, she still wanted to wear her nappy. But because we've been talking about it more (and she knows she gets a smartie if she wees in the potty), we've had a bit more success. This week she has wanted to wear the nappy more often than undies, but she will tell us when she is doing a wee and will sometimes run off to the potty even if she has a nappy on. We haven't had any accidents all week, probably more than 10 successes.

I highly recomend the Big W training undies (or similar). If there is an accident, it will catch it.

Did you have any more luck today?