IMHO, the best method of speed control is controlled linked turns. Usually larger S turns are going to be higher speed, so you want to do shorter sharper turns, really digging the edge in more to control your speed.
Check out this video, there is some pretty bad technique going on here with arms swinging, odd stance, etc. But look at the rhythm and pace of the turns. That is what you want to aim for to control your speed on a crowded hill, or narrow run, in the trees, etc.
Learn to do short radius turns and it'll really amp up your riding!
Also, don't hesitate to get a lesson. Youtube is good, but it's not a coach. Nobody made it to the olympics by having youtube coach them...