Work your way into tight areas.
Go find some trees with a good bit of space between them, where you can start getting used to the flow of picking a side and how hard to cut into each one.
Gradually move into areas where the trees are tighter together. If things get hairy, don't try and come to a stand-still, you'll wind up hitting something anyway while you're out of position, which winds up being even worse. Instead, focus on going left or right of every tree (whichever you're more confident with or whichever works with the flow you're in). If you just focus on doing that you'll eventually wind up bleeding off enough speed to regroup or come to a stand still.
And definitely wear a helmet!