Lint your code!

Lint your code as you write to aviod simple errors. Set up an automatic linting plugin for your favourite editor and kill the edit-refresh-"Syntax error"-fix cycle! A good linter will catch things like extra commas in JavaScript or unused imports in Python, or undeclared global variables that are probably a spelling mistake. HLint for Haskell is AMAZING, just so you know.

All the good editors support this in some form, including vim (http://bitbucket.org/tim_heap/linters.vim) and Sublime Text.