A friendly reminder is ok now and then. Acting like you're a mod isn't, like telling someone they're going to get banned or warned or acting smug. Just be helpful and not an asshole. Point them in the right direction, or if they sign their names, let them know they should check out the rules. Backseat modding while being a dick is what I dislike. I like it even less when people both backseat mod AND report the person they do it to. That annoys the hell out of me because then I have to go look into it and oh they've already learned from their mistake and my time is wasted.
Also, If you wan't to poke fun at an idiot, don't try to backseat mod at the same time too. And also be actually funny. That is really important.