I've tried several methods. Every child is different, so it's a bit of trial and error.
I usually keep a pull-up at night and try underwear during the day. I make the little one go at least every hour and before sleeping. I cut off drinks two hours before getting for bed including foods that are mostly water like oranges.
A girlfriend had a tea party. They drank water, juice, milk, etc all day. I did the same with my little boy. He likes to play in the toy kitchens. We got out all the play plastic food. Pretended to eat and drank a lot. Since it's getting warmer, you could make ice slushies from frozen and mashed kool-aid or frozen juice. The more they drink the more they pee. During food/drink time, take them potty about every 20 minutes.
I have tried keeping the little one in underwear. Everytime they pee in their pants, I tell him he needs to tell me when he has to go. Most kids don't like being yucky and wet, so it's a discouraging thing to have pee running down your leg.
I have put several little training potties around the house. I have one in the living room and in bathrooms upstairs and downstairs. I have the little potty seat you put on top of the regular toilet. I've found that one son would only poop if he sat on the little potty seat on top of the toilet and pee in the child's toilet. My other one will only stand pee-ing at the regular toilet. (Since kids miss A LOT, I would suggest the stand and pee method. I kind of regret teaching that one since he's so young.)
I sing a little song after they go "pee pee on the potty, pee pee on the potty". They sing along and think it's fun. I give them rewards, hugs and kisses when they go and make a huge deal out of it. I reward with stickers. They get a sticker for pulling up their own underwear and pants, another sticker for pee-ing, and two stickers for pooping in the potty. They also get a sticker for just trying to go. I bought about a 1,000 assorted stickers for about $2.00 at Wal-Mart. They are brightly colored, all sizes, designs, etc. My friend has also used kids tattoos that you put on with water if the child goes consistently all week. My other girlfriend told her son they would have a "potty party" once he was potty trained. He was so excited about having a cake, friends, and presents he was trained in a week.