I put this in another post...it seemed to be the same issue...
Have you ever watched SuperNanny? I purchased her book and here's what she says about this, "First, eliminate all excuses for getting out of bed - thirst, pee, etc. Then the first time your child gets out of bed, escort her back and explain that it's bedtime. Give her a kiss and hug. Then leave. The second time, put her back to bed and say, "It's bedtime, darling." Give another cuddle and leave. The third time, put her back to bed without saying a word. Subsequent episodes should be treated the same way. No talking, no conversations, no debate. You must get a grip on yourself and understand your emotions (the parent). You are not being mean, you are just teaching your child to stay in bed."
"It is very important with this technique that the parent who put the child to bed in the first place is the one who takes him back to bed when she wakes up. Follow through is essential. This lets the child know that he cannot play one parent against the other." - SuperNanny, Jo Frost pg. 196
She goes on to talk about a reward chart for staying in bed. If you've ever watched her show it seems to work very well, but it takes a few nights for the time it takes them to stay in bed to be immediate.
Hope this helps!