If you're vulnerable
Overcalls should promise a better suit, more high-card points and more playing strength than non-vulnerable overcalls. Be sure you have a long, strong suit, especially if you bid at the 2-level or higher.
Pre-empts (opening 3-bids, 4-bids and weak jump overcalls) should be very sound. A good rule to follow is to be within two tricks of your bid -- your hand should have enough playing strength that you expect to go down no more than two tricks if partner contributes little or nothing.
"Stretch" a little to bid your games. If you think you have the strength for a game contract, go ahead and bid it.
If you're NOT vulnerable and your opponents ARE vulnerable
Overcalls can be made with weaker hands (as few as 9 pts. at the 1-level). The better your suit, the fewer points you need to make an overcall.
Pre-empts can be "light" -- a hand with playing strength that is within three tricks of your bid (6 playing tricks for an opening 3-bid).
If you and your partner have a good trump fit, you should compete aggressively. Don't make it easy for your opponents to bid their game and win the rubber. The penalties for going down in your contract can be relatively small, but your loss if the opponents bid game are high -- 700 points for the rubber bonus, plus the value of their game.
This is usually the only time a rubber-bridge player will consider making a sacrifice bid -- a contract that you know is going down, but that you hope will cost less than the value of your opponents' game.