From the "Well, that's unhelpful" department...
Apparently, the advice from a whole load of self-help books -- to use positive thinking and affirmations to increase low self-esteem -- can actually drive down self-esteem levels in people who already have trouble thinking good things about themselves. It's a phenomenon I've observed before, but the explanation these people have for it makes a great deal of sense.
It still leaves me wondering "well, what should people do about increasing self-esteem, then?"... but I suspect that rather depends on the individual. I know what worked for me, and there are other things which work for others -- but just willing yourself to have a better self-image may actually backfire.