Lotsa good stuff here for a n00b like me. But the best single piece of advice I found anywhere has been:

Own the lane.

I forgot that one this morning and was nearly sideswiped by a minivan. Closer shave than Gillette. That particular street is busy and narrow with not enough room to share the lane if there's oncoming traffic. I dunno how or why I forgot, but I was too far right. Had I been just a foot to the left, (and I'll take two in the future, thankyouverymuch) she couldn't have gotten past me.

Wish I'd bookmarked the article or forum post with that advice so I could share it here...