Tips on getting a co-operating baby!

My ultrasound at 19w didnt go so well because s/he was sleeping on its belly the whole time and the tech wasnt able to get all the measurments aswell as the gender.. Take #2 is on thursday at the same time as the last ultrasound (when baby's the laziest) and I need tips on how to keep it awake/moving so I dont need to go for a take #3 ahah. Thanks ladies!