Comment and say something nice about the next ten fics/fic chapters you read, whatever they may be. If you want to put in constructive critism as well, good for you, all the better, but try to find one nice thing to say first. Even if it is the worst Mary Sue you have ever read, or has the worst spelling known to man (if you made it to the end of the fic/chapter it couldn't have been totally terrible), try to find one thing to say to encourage the author (and if it is that bad I suggest you do leave constuctive critism as well even if it is just "You might like to find a beta reader").
Personally I'm hoping I can make it a habit :).
