Even if smurfing could technically be punishable, Riot themselves acknowledged there was legitimate reasons to smurf and decided to tolerate the practice. On the other hand, there are clearly some abuse cases. So in order to promote positivity and sportsmanship, here are the rules I try to adhere to myself when I'm playing much below my real MMR. I'd be interested in other people opinions, and I think giving them more visibility is a good thing overall.
* First and foremost, don't be an ass. If you normally play at a much higher level, have hundreds or thousands of game of experience, of course you'll stomp your opponents. Don't brag, it's not extraordinary, it's just logical.
* Be open about it. Honestly tell your real level, so that your opponent doesn't feel too bad. And explain why you are doing it (to play with a friend, to train a new champ, etc.), so that they don't get angry.
* If you can, try to teach a little, both to your own team and to the opponent. For instance, make a dragon call, and explain why now is a good time, or ping back when someone else makes a bad call, but also explain why it's too dangerous right now. And if an opponent is making an obvious mistake and you kill them for it, tell them what was the mistake. And as per the first point, don't be condescending or bragging while doing it. Builds are also an easy thing to constructively comment upon.
* Try to show some mercy. Don't actively try to make someone life miserable or don't blow everything you have just to get that 15th kill just for the glory of it, if it's obviously not worth it. Obvious example that comes to mind is: if you play Rengar, don't always use your ult on cooldown to kill the same guy over and over again, even if you can.
* On the other hand, do play seriously, don't just fool around. Purposefully going in 1v3 is still stupid, don't do it just because you can get away with it at this MMR. On the flip-side, if you made an honest mistake and suddenly find yourself 1v3, don't let yourself die just because you don't care about the outcome of this game, and do your best to get away alive. In a nutshell, do your best, just as if it was a Ranked game.
* EDIT: as per popular request, I'll add another point. Don't Ranked on your smurf if it is below your real MMR. That should be obvious. It's not fun for the people you're stomping, and it devalues the achievement of your team-mates. It's not fun for anyone.
So that's it, people. What do you think about it, and what do you do when you're smurfing yourself?