what i do:
1 step out flat with left foot
2 step forward with right foot, but only touch the wheel to the ground
3 find your center of balance and you should start to roll
4 lift your left foot back up so you dont do the splits, put it back down (wheel only touching) and as long as your balanced and have some speed, you will roll
5 once you get better, try just jumping into a roll
i hope that made sense. (you may want to step right foot first, its just preference)

hints* keep your feet one behind the other, not side by side, even try resting the toe of your trailing foot on the heel of your leading one

Some simple suggestions:

First of all, with Heelys, one foot has to be in front of the other. Don't try to skate with your feet side-by-side.

Weight should be even distributed.

When you first start, your knees have to be locked (straight). Don't bend your knees until you get the hang of it otherwise it makes balancing much harder.

Next - if you're having trouble, wear them to the supermarket or hardware store. Hold a shopping cart and skate. This will give you some practice. Eventually, let go of the cart (little by little).

There is a "how-to" video. I have copy on my site, and I think its posted on www.heelys.com - my site is www.chautown.com ; but I run it off my cablemodem so its a bit slow.

Hope that helps.