You can ease the main traveler and it actually does have a depowering effect, but it's not fast to play it and you should only use that as a last resort, like in HUGE puffs. If it's crazy windy and you feel like you have no control, leaving your traveler down around 6 inches will help.
You should be playing your mainsheet downwind, it is your backstay, so don't let it completely off (or drop it while it's uncleated), but the F18 masts are strong and can handle the abuse. That being said, make sure your mast rotator is out, almost at 90 degrees, or just completely off, this will help support your mast better, give you a better sail shape, and use your mast more effectively. Ease your main from your upwind setting then as you get puffs ease it as your bear off, if you need to bleed some power ease your main a bit more than the optimal sheeting and it will spill air off the head. As you head back up (after the puff) sheet in again. The windier it gets, the less you ease from your upwind setting to the point that your almost just easing a few inches. If it's really crazy, oversheeting the main will also depower you downwind.
Also, not sure if your doing it, but make sure to ease your downhaul downwind, it can be tough to stay in a groove if you don't.
As you get a better feel for the boat you will learn to steer in a groove that the hull is constantly skimming and when you feel it getting too high you can bear off a bit to keep it just skimming. If you don't anticipate the puff in time and the hull does pop up, remain calm (unless your actually going to flip!) and bear off smoothly.... you will waste the puff but you will keep moving forwards fast and stay in a groove. It's not the end of the world to pop up high, but if your doing it too often you are probably heating your boat up too much in the lulls and not sailing a low enough course. Also, if your hull is consistently up or popping up, make sure your crew is on the trapeze.
Don't pull your boards all the way up, just 2-3 feet, more as it gets windier.