I pottytrained both of my boys at around 2-1/2 (one was 2yrs6mos and the other 2yrs 8mos). Both went straight from diapers to underwear. I just told them, there are no more diapers today, you get to go on the potty like daddy. They had a little piece of candy from the "treasure box" for going- something small for a pee and something they liked better for a poo (i.e. a small tootsie roll). Then they also had a reward sticker chart- 1 sticker for "trying" without success, 2 stickers for a pee, and 5 stickers (stars) for a poo. Once they filled the entire chart, they would receive the present that was sitting on the top of the fridge. By the time we got the chart filled (about a week) they were both totally day-time potty trained. You have to negotiate the nap time to fit with right after they have peed for max success and no accidents. At night I would put back on a diaper (they would cry b/c they didn't want it) but then I would put their big boy underwear over the top. Limit fluids after 5pm.
My first son was potty trained in 3 days and the second was potty trained in 1 week. The first didn't have an accident overnight right from the start, and the second was night trained in about 3 weeks.
I think part of it is that they need to be ready, but also that YOU need to be ready---ready to take the time to watch them constantly, stay home for a few days, etc. but if you can do it this way it goes much more quickly! Good luck! Oh, and by the way, they aren't ever going to tell you at first that they have to go... just set the timer and take him every 20 minutes until he goes. If he does go, then you could give him maybe another 30 minutes until taking him again. After even the first day or two you won't have to take him nearly as often because after a few successes he will start "holding it" without even knowing it. good luck!