I am aggressively non-superstitious about my dice. That's my mojo -- don't let them know you're on to them.
My players have two rituals for dealing with dice that don't roll well.
1) Holding the die 20-side up and rubbing across Gary Gygax's name as it appears on the cover of the 1E Player's Handbook. (They got this from KotD, and sometimes substitute Monte Cook's name on the 3E PHB).
2) Putting the offending die by itself away from the other dice and sternly instructing it, "Sit there and think about what you've done!"