Defending blinds too much is one of the most common and expensive mistakes in poker.
 

A big blind always has to check, except with high pocket pairs. These hands can be raised and reraised. Other hands normally worth a raise from early position (like K-Q and A-T) may be checked, because one extra bet doesn't get anyone to fold. A raise would build up the pot, but it discloses the strength of the hand at the same time. Since the blind can have whatever, it may be more profitable to disguise the hand before the flop and to check-raise later on.
 

If the pot is raised, a big blind can call one bet with all the starting hands he would call one bet in late position. In very loose games, a big blind gets huge pot odds and can pay a single bet with one-gap suited connectors (6-8), too. If reraised, he has to possess a hand worth raising in early position.
 

A small blind can call a half bet with all medium and small pairs, with at least one big card (e.g. K-2) and with all suited connectors (7-8). In loose games, even less will do, and at least one-gap suited connectors are worth a half bet. Big pairs and other pocket hands normally worth a raise from early position can be played the same way as a big blind. Also, when the pot is raised or reraised, a small blind can be played according the same guidelines as a big blind.