I would only lay my babies down once everytime I went in and would leave even if they stood up again. Not making eye contact and saying very little is really important because it takes the fun out of it for them. I was very clinical about it but it was the only way to send them sleep messages rather than letting them think it was playtime.

As Emma said, it is a phase and it will pass. Both my older kids (DS is still in the cot) lay down when put to bed and don't stand up again.