Well, to put it simply. I type the main topic in Google Adwords Keywords tool and rather than looking at results, I look for keywords which are relevant to what I am going to write.
I write them on a piece of paper or copy and paste them in word (as my mood suggests). They are usually 40 to 50 in number. This is where I look at statistics and pick only those that are relevant yet easy to target.
You may think that this process took longer than 3 minutes but I am used to it and it usually takes only 3 to 5 minutes.
I don't put much emphasis on keyword research. I have learned SEO from a reputable institute from Delhi, India and there I learned one thing above all, I create content for readers and not crawlers! :)