A) Specify what you want and your budget.

[...]

A) Do not use other peoples art work to demonstrate what you want, this encourages plagiarism.

B) Try to explain what you want is as much detail as you can, in both post and thread title, and always include what your budget is.

C) Descriptions:
.....I) Describe what colours you want, and what resolution (size) you want the image(s) in.
.....II) Avoid words like 'cool' or 'detailed', as they don't tell the artist anything.
.....III) Works like 'rough' and 'smooth' are useful.
.....IV) Styles like 'vector' or 'painted' help a lot as well.

D) You must pay at least the originally quoted price to the winning artist(s).