I started with my oldest around 2. Maybe earlier. He could pee in the toilet only if he was naked. Once he had clothes on, he peed himself. We did some reward charts for him that earned him a Thomas train and as he got better, the charts got harder. At first he had to go pee 20X to earn a train. Later, it was like 200X. We probably own like 100 trains now. :-/ And he took till age 4 1/2.
My next child I refused to train. But at 25 months she did it all on her own. Her reward was a high five. I gave her all the control and didn't try to micromanage it.
My 3rd, now 28 months, has 2 potty chairs and 2 potty seats. He has books and DVDs and we leave him alone. The other day he actually peed in the toilet all on his own! That's a great start IMO. But I'm going to just be laid back about it and let him lead me. I know he won't go to school in diapers. And I can't take the stress of trying to make him do something that I have no control over anyway.
I used essentially the same method for both of my kids. Preschool was the driving factor for both so we decided in May before they were to start to give it a shot and see how it went. My son was 3 years 5 months, my daughter was 2 years 9 months.
We trained both on the adult potty. I used a pretty laid back approach that just happened to work perfectly for both of my kids. For the first week or so we did underwear or naked at home and a pull-up/diaper when we had to leave the house. Once they were getting the hang of it, we did short errands close to home in underwear and then by week 3 they were in underwear full time. My son was completely trained in 4 weeks, my daughter in 3.
Since my son was older, a sticker chart and M&Ms worked great for him. My DD liked the stickers but enjoyed the verbal praise a lot more so she was pretty easy in that regard.
I can honestly say that I enjoyed potty training both of my kids. I didn't stress at all over the process and neither did they. It was quick and painless for all of us and I couldn't have been more proud of them. I got some grief from people about switching to the pull-ups at first when we needed to leave the house but I refused to be on lock down for potty training and I know my kids, they adapt very easily, so it just worked for us.