I've always done the treat approach...and it worked like a charm on all of my own children and my daycare kids. I've done the sticker thing too...worked with some, others got bored. I've also found that some children will consider pull-ups still like a diaper and use them as such. For me personally, I've only used underwear (pull-ups only at nap). I've plastic underwear that can pull up over the underwear too...to protect carpet etc...

If they are ready it shouldn't take too long. For me, my girls were potty trained in a week and my daycare kids took maybe a week and a half.

good luck!