I think the suggestion to look where you want to go is one of the most important for successfully negotiating downhills. Most people I know have to learn that the hard way and through conscious effort. In other words, they (and I) looked at the tree they wanted to avoid and then BLAM!... hit it! But if you make a conscious effort to consistently look where you want to go, it will become second nature to do so. Nowadays I rarely look at what I want to avoid and it works... the times I do look, I usually do run into it!

That's why it's important to look a little ways ahead rather than right at your front wheel. From further away you can scan what's ahead, recognize what you want to avoid and then choose your best line and take it.

I hope that helps you as it did me when I finally "got it!"