Are there any grayhat (but not illegal) ways to improve credit score?

I'll make this brief. I have depression–it comes and goes–and it was during one of these bouts of depression that I let a $500 Capital One credit card go to collections. Stupid, I know. But if you have ever struggled with D or know someone who has, you might understand :p. I've since paid the card off, but the damage is done.

Anyway, that's ALL that is on my credit record. I would really like to get my credit score up. It's around 530 right now.

I know that I should get a secured card…

Are there any grayhat (but not illegal) ways to improve credit score?