This WordPress plugin is free, and makes tagging your posts a snap. It’s a HUGE TIME-SAVER.
After it’s installed, you’ll see a checkbox labeled “Autopopulate Tags” just above the Publish or Update button on the post editing page in your website admin. Simply check the box, then either click “Save Draft”, “Publish” or “Update”.
The plugin will scan the title and content of your blog post and compare it to your entire list of keyword tags. If they are present in your content, they’ll be added to the post automatically. You may then either delete irrelevant tags, or add new ones, which will be part of the next scan you run.