I was reading on-line for things to help me with my daughter. She is 3 and I didn't have money for pull-ups so I was desperate. I was very tired of waking to change the bed every night. Anyway, what I found was:
1) During the day when they say they have to go don't always immediately take them. Let them hold it for a few minutes. Their bodies can get into a routine of feeling the sensation and almost immediately getting it out. Therefore, that is what it does at night.
2) Wake them up about 3 hours after they go to sleep to go pee. This was actually controversial. One website said it was a great idea and the other said then you are teaching poor sleep habits. I did it with great results. I thought having to change her sheets nightly wasn't teaching great sleep habits either.
My daughter will now wake me up when she has to go pee during the night. She doesn't have to go every night so we don't have to wake up every night now. These are just a few things that worked for me! Good luck!!!