Now that I've told you not to potty train your kids too early, let me give you a couple of tips that made all the difference when I potty trained my daughter. They won't work for everybody, but they were the right thing for us.
First of all, don't over-praise. The more you make a big deal about potty-training, the more "power" the child has and at that age they'll use every bit of power they can get. If they know you want them to do something, there's a good chance they won't, just to be contrary.
I did try letting her wear just underwear for a day. That lasted about an hour, until the first accident. She pointed at her puddle on the floor and ordered me, "You clean it, Mama." Um, right.
What actually worked much better was putting her in snug fitting leggings over her panties. When she had an accident the leggings absorbed everything. Then when she told me she needed a change I told her, "Just a minute," and pretended to ignore her for about 5 minutes. She hated the feeling of the cold pants and didn't even want to move. Instead of me making a big deal about it (she loves any attention, even negative), I just told her she needed to wait until I could help her.
Try expressing some doubt in their ability. Kids love to prove us wrong. My trick was to get her stuffed animals talking about it. The bear would tell the bunny, "I don't think Olivia is big enough to sit on the potty." And the bunny would tell the bear, "I think she can get up there, but I'm SURE she can't go pee-pee."
This next trick STILL works for me. She loves to race, so all I have to do is say, "I'm going to go potty FIRST," and then I start running for the bathroom. She practically runs me over trying to get there ahead of me.
And now that I've said all that, I'll just reiterate the best tips: Don't start too early, and take your time. Having a potty trained child isn't any less work for you as a parent, it might even be more. So don't rush it, just follow your child's signals and keep reminding yourself that we all figured it out eventually - your kid will too.