We used a chart both times, but it was all a bit jazz after the first couple of days - some of the stickers on their t-shirts, some of them on the wrong days yadda yadda. And then it all got forgotten about quite quickly. No chocs/rewards apart from that though, but we got ds2 a big pressie (he was desperate for a bike) once he cracked it. He immediately reverted to pooing in his pants for a few days once he got the swag. This is more a reflection of ds2's personality than anything else. He's a bit fly.
I have used stickers and it seems to be working but then my dd adores stickers. we made a chart together simply poos on the left wees on the right with lots of scribbling and colouring all over it. She likes sticking the stars on after she has been on the loo and counts the stickers every now and again. Whether it's helping with the training I don't know but she likes it.