DON'T try to go in a straight flat line. You've gotta be constantly cutting into the snow unless you're building up speed for a flat section. Otherwise you will fall and fall hard.
Put your weight into every turn and cut, it'll make all the difference.
If you need to slow down, dig your heels or toes in harder and turn the board perpendicular to the slope.
It takes a while to get the hang of it, it's more of a learning curve than skiing (although I've never skiied, just heard that), and you will fall A LOT at first. If you keep up with it though, it'll be worth it.
Seriously wear a helmet.
Make sure your pants and coat are actually waterproof, and that your gloves are well insulated. Maybe it's just me, but my hands get colder than anything else.
These may sound like no-brainers, but when I first started some of them took me a while to get.