I was going to suggest what Marydean said about the backyard.
Darcy started doing that about the same time - but it was always after she had done whatever, she just couldn't manage to know before so i left it. Eventually she just didn't want to wear a nappy or undies so we left her at home and she got more confident.
It really only took 2 days after she'd figured it out. That was about 10 weeks ago and we've had maybe 6 accidents since then.