How to get your baby to cooperate during a sonogram?

i am 18 weeks pregnant, tomorrow i am finding out the of my baby, but i am nervous! a lot of girls i know, went to find out what they were having & the baby kept its legs closed the whole time. i was wondering if anyone had any tips like things to eat or drink or ways to move my body in order to keep my baby moving! or what to do if the legs are closed?