Let say you have a website on bikes.  What you  do is go to Google and punch in push bike.

Now, what other keywords do you think will come up on the first page of Google related to push bike?  You might see cycle, mountain bike, bicycle,bike videos, photos, or cheap push bikes,push bike forums and reviews.

You use these keywords and tie them into your content.  You know those are the kind of keywords that people are searching for.