What is the best way to hold in your tummy?

There is no way until you train the muscles under the six pack and the only way to do that is to put your finger in your belly button (or hovering just in front of it barely touching your skin) , use your stomach muscles to pull your stomach away from your finger - hold for a count of 7, let go. Do 3 sets of 7 each day - in less than a week you'll start to notice a difference.

you can always try a body glove like Kymaro - my friend has one and swears by it.