I initially treated the Small Blind much like any other position -ignoring that I'd put in already. Then I tried to factor in that often if you only have to put in a half bet, you get reasonable odds for most hands, and ended up playing most hands. The 2nd method worked out worse than the first, because it meant putting money in with too many mediocre hands. The balance is somewhere between - small pairs and suited connectors and the like are more worthwhile playing, because you'll either hit the flop well or not nearly enough, but weak AX hands aren't nearly as useful, because when you do pair your Ace you're in the wrong position to know where you stand, and your positional disadvantage is probably more significant than the advantage of playing cheaply.
In early position, you have to be more selective about your hands - in the Blinds you'll see a few more cheap flops with 'fit or fold' hands.
In the Big Blind, if someone looks like they're trying to buy the pot with a minimum raise, then you want to defend it most times because the odds support calling with most random hands, and if someone always tries to buy your blind, then you have to defend it sometimes at least (otherwise they'll keep doing it), but you don't want to let that logic get you putting a lot of money in with a weak hand, especially when the aggressor will have a positional advantage over you through the rest of the betting.
