I've already pointed out at least one keyword related hidden gem already. But the ability to create well thought out - and targeted - keyword lists is so essential to success with AdWords, that I thought I'd point out a couple of other useful pages as well.
First off, it's difficult to create great keyword lists without first having given some thought to your actual advertising goals. Here are a few essential questions to ask yourself, that'll help you decide how best to organize your advertising, and create effective keyword lists.
Then, an AdWords Help Center page entitled "Where can I find keyword ideas?" provides a couple of very simple tips which, over the years, I've realized that lots of advertisers simply forget to leverage - perhaps because they are simply too close to their own business.
Sometimes it pays to just take a step back, and focus on the basics!