So it's worth constantly trying to put yourself into the minds of users. Keep trying to think what they're looking for, as well as how they might be wording their queries.
Obviously, you can't use intuition and common sense for all of this stuff. You have to have some data, for example the search volumes for various keywords. That said, there are many things you can figure out just by thinking about them a bit.
You can learn a lot from your own behaviour, too. So it's a good idea to keep mental notes of searches you yourself are making. Examples include how often you use queries in the form of a question and whether you include geo-specific keywords. You'll start to see some trends emerging in your own approach that can give you some good ideas about the kind of terms other people are using.