This is an great topic for a longer post in the future - but for now, here are a few headlines about disabled keywords.
* As a matter of routine, if you see a disabled keyword (one red *) in your account, delete it. This is important to do, regardless of whether you plan to re-use the keyword or not.
* Try to analyze why the keyword got a low CTR before re-using it. Usually it is simply too general. Consider making the keyword more specific. For example if 'widget' is disabled, delete it, and try 'blue widget' or 'blue reverse widget' instead. Usually, the more specifically the keyword describes what you have to offer the better. And the more highly related the keyword is to the ad that appears, the better your CTR will be.
* In order to re-use a disabled keyword, delete it first *wherever* it appears. Then put it in a new Ad Group and give it an ad that is very highly related. Put the keyword in the ad title, if possible.
* Consider making the keyword a phrase match - "tiny widgets" or exact match - [tiny widgets]. This is likely to limit the number of impressions, and may improve the CTR. Also, this is seen by the AdWords system as a new keyword, and it will be given a fresh start.
There is more to say but this post is *way* too long already. More later! :)