In the area where I race at, the way we found to get someone to move up a class is to insult them with their petty win and constantly ask them, "Ok who did you beat? Were you in the Mod Class(we view this as the premier class, win it then your fast)? Then it doesn't matter." If enough people constantly rag on the few people who constantly dominate those lower classes, then eventually their win will mean nothing to them and they will move up on their own. It has been affective in getting sandbaggers out of the sportsman classes. Again, just my opinion and experience in the racing schene around where I race.
disclaimer: these people we pick on are our friends, not new people to the hobby. We do not act this way towards those we do not already know well enough to do this to. We try to keep it fun for everyone.