B) Try to explain what you want in 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) Words like 'rough' and 'smooth' are useful.
.....IV) Styles like 'vector' or 'painted' help a lot as well.