Andrew,
Follow these steps (mostly stated already) and you will pop up every time.
1. Sit in the water with your knees crunched up a fair amount towards your chest.
2. Let your arms dangle loosely around (or on) your knees, holding the rope.
3. When the boat begins to pull, don't try to stand up, stay semi-crunched and keep saying this to yourself over and over..."Put my heels to my butt, put my heels to my butt"
4. DO NOT PULL with your arms! The boat will always win. Let the boat pull your arms, do nothing except "Put my heels to my butt."
5. Once you pop on to the water, take the handle with both hands and swing it to the hip that you normally have pointing towards the boat (left hip for regular, right hip for goofy).
6. IMHO, get up before you try to leave the perpendicular position...don't think you have to yank that handle to your hip instantaneously...heck, you can ride around the lake doing a boardslide (although as a newbie you unfortunately would probably catch a nasty frontside edge somewhere along the way
Let us know how it goes!