In addition to what's been mentioned:

• Don't ignore error codes - e.g. don't assume that you got a valid result, that a file has been successfully created, etc... Because some day, something will happen.
• Don't assume that your code will never enter some condition and that therefore "it's safe to ignore that condition".
• Test your code, then have it tested by someone else. I find I'm the worst person to test my own code.
• Take a break, then re-read your code and see if you "missed the obvious". Often happens to me.

Lots of other things I'm forgetting at the moment, but the others will surely think of them. :)