Keywords are very useful for relating content within your site which then has benefits for usability and SEO. I define keywords on this site at folder level and then related folders show at the bottom of the pages since I call portlet_related at the bottom of each folder_listing. For example, see /SEO-Best-Practice.

• To define or Add keywords simply add them using the Properties tab for each document.
• To manage keywords, it is best to have a single document, e.g. keywords, which has all your keywords, then you can add or delete them from there.
• To delete keywords, at the time of writing, there is no tool in the ZMI, so you simply remove reference to them in all documents. If you do an advanced search on a keyword, you can see all documents which contain them. If you have a single document containing all keywords, simply go there and remove reference to that keyword.
• It helps not to have too many keywords and not to define them on too many documents - I use them mainly for top-level folders.
  
• Use keyword terms which you can later refer to on pages on the site, i.e. not shorthand, with sentence case.