Maybe a sticker chart??
Some sort of reward system?
With my son, we rewarded him with a super bouncy ball every time he peed in the potty. One for every time he peed. When he pooped in the potty, he got 2.

However, I will say that for a good 3-5 weeks, he had tons of poopy accidents. He wasn't refusing to go in the potty -- he was trying. I think he was still trying to figure out what his body was telling him.
So, we had the bouncy ball as an IMMEDIATE reward for the potty. We had a sticker chart hanging in his bathroom -- he would get a sticker each day for being a good 'toileteer' (stole that term from Bear in the Big Blue House, term used in a book about learning to use the toilet and how Bear helps his friends learn how). So, each night before bed we would discuss if he had been a good toileteer. Good toileteer didn't mean NO accidents. It meant you did the BEST JOB possible. Accidents were ok; but purposely pooping on the floor was not. KWIM? If he had done the BEST JOB possible that meant he was a good toileteer and he could have a sticker. After 5-6 stickers, he got a reward (new box of crayons or new book; one prize was a Dora cup b/c he loved Dora at the time). What I did with the sticker chart: I used poster board and drew5 or 6 rows of 6 or 7 squares across. The last square was for a picture of the BIG REWARD. (Either drew a rough picture of the reward or cut part of the packaging and taped on that square) So, he could see the progress he was making toward the picture of the BIG REWARD.
OK...so that is my 2 cents! GOOD LUCK!