DEBUGGING! And I don't just mean echo'ing out statements to check the values, I mean an actual attached debugger to your code. GDB for C / compiled languages X

DEBUGGING! And I don't just mean echo'ing out statements to check the values, I mean an actual attached debugger to your code. GDB for C / compiled languages XDEBUG for PHP etc The actual act of stepping through your code and looking at the values, datatypes and their transitions massively increases your productivity if something is tricky!