tedster - 5:46 am on Dec 10, 2007 (gmt 0)

My aim is "not at all".

1. the keyword metatag has not been a part of the Google algo for some time
2. the description metatag is used to compose snippets - but again, it's not apparently used for ranking purposes at present.

Because of this, I try to write a good description - one that "attracts the click" - on the first shot. I only change the description if something looks really wonky on a SERP. This can happen if a page starts to rank and drive traffic for a search that I didn't plan for. If that keyword or phrase isn't included in the meta description, then Google might assemble the snippet from other ingredients. In that case I may re-write the description to gain some control of how the result looks to the user.

If you're concern is improved ranking, frequent changes to meta tags are not the best place to spend your energies, IMO.