I just got Harrington on Cash Games on my lunch break and while reading the first chapter I came across this concept I never thought of before and I hope you find it useful. Food for thought....
"Fold or bluff with your weakest hands. Folding weak hands is obvious. Choosing your weakest hands for bluffing might not be so obvious, but the basic idea is that if you bluff with a weak hand and the bluff works, you've gained value from a hand that had none. If you pick a hand with some value (like a medium pair) for bluffing, but then get raised off the hand, you've lost value with a hand that had some to begin with."