It is best to plot out how you'll use meta tags on a particular page before even writing the content for the page. Lots of people will write the content of the page and then try to sprinkle in keywords in the meta tags. This is an ineffective way of doing things, as it usually makes your page confusing to visitors.
Before writing content, take a minute to pick out 3-4 primary keywords. Then take another minute to write phrases from those words. Once you've done that, you can begin writing your content around the keywords.
How do I select proper keywords to enter into the meta tags? This is an excellent question. Really, there are two main rules of thumb for selecting proper keywords.