Ohhhhh, sigh.....feel your pain.... Two things that helped with us (my daughter will be 4 next month) were what I call "Playing Stupid" and "Line Leader". The first one is going up to your child and saying, "Oh! I feel something in my bottom that needs to come out!" and having him/her lead you through the steps of pottying. The 2nd one my mom figured out the last time my daughter spent the night w/her (because up till a month ago we were still having issues..!) -- she would say, "Well, I guess I will be the line leader to the bathroom...." and of course my daughter HAD to be the one to lead! Ha ha! I kept it up at home, and when we'd get to the bathroom she'd want to run off and I'd say, "The line leader to the bathroom is the line leader on the potty." And she'd get on up there! Now she goes a lot of the time alone without even saying anything to us, and other times she'll tell me that she needs my help.
I'm not saying that those things are the Magic Key, because it's just whatever clicks with that particular child. Just be creative and try and think of off-the-wall things that can tie in to pottying. It will not be an overnight thing but if you stick with it it WILL happen!!