Thesauri and Authorities
The Library of Congress Authority headings for Subject, Name/Title combinations http://authorities.loc.gov/ this source has a long lag in updating. You should not rely solely on it but should verify the source carefully.
The Library of Congress also maintains a Thesaurus for Graphic Materials http://www.loc.gov/pictures/collection/tgm/ This list comes in handy when describing 2D materials – photographs, prints, designs, etc.
The Getty Research Institute maintains the Art & Architecture Thesaurus online http://www.getty.edu/research/tools/vocabularies/aat/ This list is hierarchical in nature and is intuitive after seeing how it is organized. I have used it after realizing that: crafts (art genres); installations (exhibitions); were from the list. Getty maintains a number of lists in addition to the Art and Architecture thesaurus: Geographic Names, Cultural Objects, and the Union list of Artists.
The Visual Thesaurus is a fun to use http://www.visualthesaurus.com/ using it can help you find the ‘right’ word to use. When you get stuck looking for a term – this is the place to go!
The Language Toolbox http://itools.com/language is a go to source for word meanings, translation tools, idioms, quotes, specialized vocabularies. Think of it as a jumping off place.
Google https://www.google.com/ is an essential tool! Don’t be shy – use that massive search engine to check your spelling, names, dates, whatever you are looking for, Google will help you along the way.
Using controlled vocabularies makes finding aids/databases easier to search. Less guessing and more description. You need to find the balance between vague description and precise description. The former will return on almost any search, the latter will be returned less often and possibly will not return a useful result in a search.
Many online databases ignore common words like ‘the’, ‘a’, and ‘this’. The most important thing about digital asset description is Who, What, Where, How, Why, and When of the digital object. If you can describe the context, then you will have gone a long way towards making the asset retrievable.