Finally got there with ds and dn recently (now 2.11 and 3.2), took about a month each to really get the hang of it.
The crucial bits seemed to be: will they sit comfortably on a potty/toilet - ds wouldn't before as he was too small for most of them.
Can they pull their own pants/trousers down? We tried when ds was 2.6 and found he simply couldn't and got hugely distressed. A couple weeks of pullups and mornings of practising up/down really helped.
Then we went out to buy COOL PANTS - Thomas the bloody Tank Engine is finally useful! And then told him the plan. He was miffed, to say the least.
The vital part was he was now old enough and motivated enough to understand bribery (peer pressure didn't help at all and he doesn't want to be grown up).
So a chocolate button/smartie for every wee/poo done on the potty (after a couple weeks, refined to wee actually IN the potty! Willy points down...), a sticker on a chart (actually sticker on him, tick on the chart) for every day of reasonable cooperation, and after 20 stickers, I would order a scooter.
Now he doesn't get chocolate for wees as he has a scooter for being so good at weeing - as he tells me smugly regularly! And using a potty (or toilet at nursery, but he's too small even with a seat and step to use the one at home) has really solved his chronic constipation problem!
Even Grandma was impressed when she took him out for the day last week - he asked for the potty every time, they trotted off to the Ladies, he weed neatly and washed his hands!
So based on my sample of 3 (dn was similar only motivated by copying ds, neighbour's son then visited and wanted to copy both), the key is creating motivation to learn and reassuring them it takes practice.