There are many good resources devoted to photo keywording. This one on the site Photopreneur discusses the importance of keywording for photographers submitting images to stock photo companies. Since the keyword guidelines for different companies vary slightly, it is crucial that photographers understand the guidelines and develop a system that incorporates all the parameters.
Applying keywords just once will be less time consuming and finding ways to automate and streamline the process is key. Another great resource is on one of my favorite sites, Controlled Vocabulary.
The Caption and Keywording Guidelines list questions you should ask yourself regarding the “aboutness” of any image. This list should assist you in creating a process that will lend speed, consistency and accuracy to your photo keywording efforts.