1. Taunt Punish: I consider it a cheap shot to attack an opponent while they are taunting. I've seen opponents respectfully veer off their attack and hop away to avoid attacking during a taunt, but I've also seen opponents charge in for a small boost of damage. Be the former!
*EDIT: How you use the taunt matters, of course. The taunt I mean is the pre-game salute. When someone hits you during a greeting-taunt, SAIN.
2. Rinse Repeat: It's always easy to break an opponents pattern, but too often I'm stuck in disbelief that a character will repeat a combo to win. There are over 1400 moves in this game to enjoy, so using a maximum of six at all times is a little silly. OR VERY SINFULLLLLLL.
3. SD for the Game: Play to the end! Hopping off the edge isn't fun for anyone! (Leveling the stocks is a different story)
4. Trash Tag (For Glory): Trash talking, but through the tags during a For Glory match.
5. Roll, don't run: I saw a Sonic, the fastest character, do nothing but rolls to move across a flat stage. SIN.
6. Prolonged Toying: Treating your opponent with respect fosters good sportsmanship and a decent reputation for the community. Don't overdo the teasing, juggling, trapping moves when they aren't needed. That being said, a little styling is fine. Practice makes perfect, after all.
7. Counters: Super Smash Diner. They have nothing but counters to offer. Of course, grab 'em, squeeze 'em.
8. Teabagging: Consider the tone of the match. Is teabagging going to be received well? No? SUN.
I'd love to hear other ideas for Smash Sins!