Thought I'd share this video Raceboy77 made with tips on how to race cleanly:

It's a long video, but really good as they do some 1-on-1 demonstrations. Some of the things he talks about are strategies I've used with great success myself. Braking when crashes happen (although as he shows, someone might decide to go all Days of Thunder and plow into you), but mostly reliquishing the racing line when passing attempts aren't successful. As he talks about, if you're not further ahead than someone's door when coming to the corner, give it up and tuck in behind. It's so easy and makes the racing so much cleaner.