Keep Keyword Selection Simple ...

... just describe the information that is on each page.

Use three search phrases, like this:

Let's say you have a page with money saving ideas.

I'd do these three search phrases:

easy money saving ideas, painless money saving ideas, how to save more money

Never add unrelated search phrases/keywords to your meta description.

Describe the contents of your page as accurately as possible, and try to think of the words the search engine user will enter on Google or Bing or Dogpile, to find your site.