Keywording is a crucial point in a post-processing process for images, particularly for images intended for the stock market, and I think that beautiful pictures will have more success if they are supported by a proper keywording.
This is why I spend some time to choose keywords and evaluate their potential implications.
Doing this, I consider three different levels of keywording:
1) LITERARY Level: it is the starting point, the keywords used hereby are useful to describe what is the content of an image (people, animals, objects, actions, events...). I base my selection on a hierarchical scheme of a tassonomic criteria, without exceeding in the excessive implications of a thesaurus.
2) CONCEPTUAL Level: this is a more subjective step, as it deals with the selection of ideas and adjectives related to the content of the picture.
3) STYLISTIC Level: it is quite a formal level, as it gives technical information concerning the scene (landscapes, portraits, posing, close-up...), the shot (vertical, horizontal, diagonal) and other stuff such as the type of photo (color, B&W, IR...), the author and so on.
Applying such criteria I am reasonably sure to produce an interesting keywording, avoiding common mistakes.
Have fun guys!