Here's my rule:

If there is an A on the board and I can't beat a pair of Aces - I refuse to invest a lot of money in the hand. If I make a small bet and get played back at, I'm probably setting it down.

If there are two A's on the board, I'm definitely not making any substantial investment in the hand, unless I have a pocket pair and end up with a full house.

Likewise, if there are 3 to a flush on the board - I won't lose a lot of money on a straight. If the board is paired, don't go crazy on a flush if someone is giving you a lot of action, especially if you find the pot is 3 way at the river.

On the other hand, you need to get more value out of the raggy flops when you hold top pair, etc. It's all about knowing which flops are safe to proceed with your hand and which flops aren't. Don't pay this guy off when the flop is bad for you.