we've used a (home made) reward chart for max for a couple of months now (since he was just over 2 years old), to motivate / reward / encourage TT... but we've extended this to include other random things like excellent efforts in eating new foods.
the system is 5 stickers = reward. the rewards are small things like a sheet of 'fancy schmancy' stickers, or a bananas in pyjamas single use bubble bath....
the system itself works well, max understands the concept of putting the stickers on the chart, and when a row is complete, he gets a reward.... the only 'issue' lies in the fact that he's simply quite happy in his nappy right now...