If you ignore the rules, people will, half the time, quietly rewrite them so they don't apply to you.
Some play by the rules. Always use this against them.
Never give a mundane explanation when a magical one can be hinted at.
If fair means have failed you, it is time to resort to foul.
If foul means have failed you, it probably wasn't worth having anyway.
If at first you don't succeed, hide all evidence that you tried.
Winning isn't everything. Your image is.
If no one saw it, it never happened, it wasn't you, and they can't prove it.
Three professionals no Slytherin should be without: a lawyer, a PR rep and a damn good style consultant.
If fortune has presented you with an unexpected opportunity which you don't really deserve, the last thing you should do is correct the mistake.
NEVER play little old ladies at cards. EVER.
NEVER seek to rob, con, beat up or otherwise abuse small, harmless-seeming little old men.
The truth is a terrible and dangerous thing, and should be avoided if at all possible.
If someone has the power to completely disarm you, to take you apart from the inside out, to turn their dreams into yours, to invade every single part of your life and marry it with theirs so you don't know whose is whose anymore, to bring you sobbing into their arms like the child you've always denied ever being and to destroy you utterly with a single glance... for gods' sake, don't tell them!
Keep smiling. A well-planned smile can be more frightening than a single glare.
Your real feelings are a precious thing indeed. And they should be better guarded than Azkaban.
Of all the Slytherins you will ever meet, there is one who you should fear above all others. And that is the Slytherin Who Still Has A Cuddly Animal And Is Not Afraid To Show It.
Bright colours are the devil's work.
Don't underestimate the benefits of Muggle technology