I'll start and let you know what I currently use.

1. I map out on paper my ideas, I either write a spider diagram of words or I try to story board with simple line drawings.

2. I then either use these directly and talk my ideas through with the artist. If they are on board and understand then that it.

3. Say the feature needs more logic or is not fully defined I start a google doc, where I take a picture on my iPhone of the art and then I use each slide to talk about one aspect of the design.

4. I then open the doc up to other product or management people who contribute.

5. When we are all happy I invite the artists.

This works OK, but I feel it could be better? Any tips.