Hi M.-
My daughter also takes Miralax, and the big thing you have to be careful of is pooping in the potty becoming a power struggle, because she may start withholding, which will be a much bigger problem. Just be really nonchalant about it and REALLY praise when she goes #2 in the potty.
We did use the reward of one M&M for #1 and 2 M&M's for #2. Eventually we phased out to 1 M&M for #1 and then none because she was a big girl who was an expert at using the potty.
We tried to keep books by the potty and encouraged her to sit on the potty- kids that age are just off and running and simply don't want to be bothered by sitting. LOL
In fact, we were recently reminded of the need for Abby to sit and relax- she got so backed up we had to do an enema. It caught us by surprise, because she was still pooping every day, but not getting it all out at each sitting. The enema wasn't nearly as bad as I thought it would be. No fun, but not too bad, either.
This is probably all too much info, but hey, you asked! LOL
J.