My DS started to show and interest early on, so we ran with it (sorta). At 18 months we got a little toilet. We asked him if he wanted to go, and if he did, great. If not, no worries. We usually asked in the morning and before bed/bath.
Kept doing this even while we lived with my parents for 3 months over the summer. He did it more and would sometimes tell us before going. He particularly liked to go when my father was going. At the time, my dad was using a walker (broken hip in April), so DS always heard him walking to the bathroom and would beat him there. LOL!
I digress. At the new house, we kept the same momentum but "kicked it up a notch" after DS turned 2. Asked him more.
Then I got some cloth trainers (snap-ez) to use at home after the holidays. At nursery school, he uses pull-ups. (dislike pull-ups and huggies brand, but... Last week, the teachers said that we should start bringing him in with regular cloth trainers. So, I think we will finish off this week with the pull-ups, then go with old-fashioned gerber trainers -- I've spent too much on the good stuff! LOL
Anyway, at home, we do a lot of commando time. It really helps though DS did pee in his chair the other day. I also bought two big packs of stickers -- one set for upstairs and one set for downstairs. We have several free calendars, so we put one in his bathroom and one downstairs. He gets a sticker for each pee, and 2 stickers (or a big fancy helicopter one) for poos. He LOVES this. DH and I are both overweight, so food rewards would set a very bad precedent in our home -- plus I have no control and would eat all the sweet stuff.
Your son has to be ready and show an interest or else it will really backfire. That is what happened to my nephew. He was sooooooo stubborn and my sister and BIL pushed so much that he would refuse to go at all. He would get constipated from holding it in and then a whole domino effect. He wasn't potty trained until almost kindergarten and would still have accidents at school if he was mad. Bad situation all around.
What I want to know is how the heck do you get them to stay dry at night. DS's diaper is SOAKED by morning.