F.P.
I don't know if you've ever watched super nanny, but she says that you should keep it short and only say things like "it's bed time", "Get back in bed", "it's not play time" ect. and only talk 3 times after that you shouldn't say a word just go in and put her back to bed and leave. But beware it can turn into a game. Now I've seen it work on T.V. for her, it is a long process. Another thing you can try is not going in there or saying anything at all, just watch her were she can't see you and see what she does, she may get back in bed when she's tired and go to sleep on her own or just fall asleep on the floor, which then you can put her back in bed. What worked for me was reading a bed time story and singing songs (in bed). We started around 1.5-2 yrs. old and to this day(he's 5yrs. old now) we still read a bed time story and sing songs before bed. It was routine he new what to expect and in relaxed him a calmed him down for bed. But my problem wasn't him getting out of bed, it was him crying and not wanting to go bed in his bed, not to different from your problem. i don't know if it will make a difference or not.