One approach is to invent a mnemonic system for constructing secure passwords. The passwords should be systematically related to the specific site (including variables that are only known or relevant to you), but the patten should not be obvious to anyone but yourself (and memorable enough that the passwords & method of constructing do not need to be explicitly recorded). This makes it possible to have a unique password for every site. As a further foil to anyone who might somehow crack the pattern, use different mental password construction algorithms for sites with different levels of perceived security importance (although then it is easier to get confused).