Having spent some time researching how to keyword effectively, Peter Krogh’s DAM Book Guide to Organizing your photos with Lightroom 5 has convinced me that the Keyword List is the best method to use. You can assign keywords to images by dragging them onto a keyword. The keyword list will let you know how many images have a particular keyword and which images do not.
In Lightroom 5, a good workflow for adding keywords would be selecting shots and adding them to a particular keyword set by selecting the checkbox next to the keyword that is appropriate. The Keyword List Filter as Peter explains is a great tool if you want to search through your keyword list and remember where you have put something. I found this aerial shot of the Opera House using this method. As a result of the DAM Book Guide which I can thoroughly recommend, I have created my own keyword list which I now use regularly when I license stock images of Sydney.