What I mean is, don't add cards about things you don't really understand or answer them too optimistically (as in "correct" after you took 30 seconds).
When studying for exams, I've had moderate success with adding e.g. algorithms or formulas before I understood them. On the first time when they come up, I don't even try to recall them, but instead look at the answer, try to figure out how it works, and then mark it as failed.
Say it was an algorithm. When the card comes up the next time, during the same day, I let myself mark it as correct if I remember the rough outline of how it worked. The next day, implementation details. I want at least one day where I can pretty much write out the whole thing from memory, even if this took, say, ten minutes. That's to ensure that I've actually understood it and I'm not passing over some fine detail that's trickier than it seems. After that, I mark the card as correct for as long as I can recall the rough outline of its function.
Admittedly, this is rather mentally tiring, and I've ended up deleting several of those cards afterwards. Working this way let me remember them pretty well on exams, however. To some extent, it teaches me to re-derive the algorithms / formulas as needed.
I have made some significant progress in organizing myself with org-mode (basically a really well thought out emacs outliner) - consider this a plug :).
Now I think I am ready to bite the bullet and automate another part of my mental apparatus, memorization. I'd like to hear other people's experiences with SRS - spaced repetition - (negative, too), what software they use, what do they use it for, how much time they spend. I expect these to vary, so stating your reasons is worth an extra upvote (and thanks ahead)
ETA: When do you decide something is worth memorizing vs. putting it into a searchable database?