Maybe this should be one of the basic topics, but I see plenty of "experts" making this mistake. Write readable code! Use meaningful variable names. Yes "foo" and "bar" are cute names but they don't say much about what is going on in your code.

Obfuscation and golfing are fun diversions, but don't write your production code that way. Use comments and spend some time on the layout of your code to make it presentable. Be proud of your work.

My $0.02.