Here's a guide that gives you some more sites that can help: http://www.seomoz.org/article/keyword-research-guide
It's a little dated, but most of the stuff on there should still exist.
Personally, some of the strategies I used are:
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Look at my analytics. Where are my visitors coming from? Are there any similar terms using synonyms or different word order that I need to target?
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Look at competitors. What keywords are they targeting? You can find hints by looking at their content, title tags, meta tags (if they're well-optimized, that is).
I think another important question to ask is Am I ranked as highly as I can be for the higher-volume terms? To me, I'd rather effectively target the terms that the most people are searching for rather than attempt to target a lot of low-volume long tail terms.