We just had a speaker at our guild last night talking about these. A couple of points I found interesting - don't use black (they see that as a 'hole' and will avoid it). And don't use yellow - that is one of the last colours they recognize and is generally used as 'caution' in the homes. Make it like you are making for a child - nothing that can come off, nothing that looks like candy (buttons, etc). And nothing hard/sharp as they may swing/throw them when agitated.
So many great ideas out there, and it is such a wonderful service for them.