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Don't just write comments above/next to/in place of unneeded code
Old comments are as bad as old code for hanging around; no one wants to be the guy who presses delete, just in case it turns out to be helpful 6 months from now. I work on several projects now that are jammed full of "JL 2012-02-26 Not needed anymore since ..." type comments, and they do nothing but get in the way (especially the ones referring to code that has since been removed/uncommented/etc, as the messages are then just damn confusing). These types of comments are exactly what version control exists for; why insert dated comments all over your code when version control dates the code changes for you?