Don't Forget the Obvious
Over the last few weeks we've been talking about a process for debugging software, but I realize that I've glossed over one fundamental step:
Don't ignore error messages!
Make sure that you have access to the logged output of your application, whether that's a browser console or a server log file. When something goes awry in your application, there is often (though not always) valuable information to be gathered from errors and warnings that get logged. If nothing else, Googling the error message is often illuminating.
And for that reason, you should also make sure you're keeping your logs clear and not cluttered. If you're logging output as part of your debugging process, make sure that you remove those messages when you're done, or your logs will be too messy to make sense of.