When we potty trained our kids (now 17, 18, and 21), we used the same method. First, they were all old enough to understand the concept, around 2.5 yrs. old. We took them to the store and had them pick out the panties they wanted to wear.(it is important that they choose) We made a 'big deal' about this, and showed off the underwear to them, even letting them try it on, etc. Then we said, 'now, we don't want to get (whatever is on the panties, as my kids picked 'Mickey Mouse') Mickey wet, he wouldn't like that, so we are going to wait until you are ready to wear them....so after one week of no accidents you may put on these SPECIAL (making a BIG DEAL) underwear!!!' You've got to make it a really big deal!! Then place a calendar somewhere so they can see it, and put a sticker on each day that they DO NOT have an accident, making a big deal about it, and showing the underwear, etc. For us, it was important not to allow them to wear the underwear and just have accidents.
I will say that all 3 of the kids were trained in 7 days, not one accident even at night! Thus, I pass this along to others. Hope it works...
Oh, and another suggestion is with a CLOSE FRIEND, allow them to see them use the potty (no I don't condone watching people in the bathroom, but a cousin, or close friend, one time isn't going to hurt them), with NO DIAPERS. They usually will question, or at least say, 'no look at Alex, she is a big girl...' and you say, 'but you and Alex are the same age, isn't it neat she wears big girl pants.'(don't say it to make her feel guilty, but encourage her to 'try it'.) If the child doesn't WANT to use the potty, this may help her to understand that her friends are doing it, giving her some confidence; but if she is not ready, this probably won't effect her...