never found inside van graphic that good a return 
outside ones seem to work well enough
Seriously just put your name phone number and what you do thats about all you can hope people see clearly
If something stands out like a bulldogs wotsits, i.e. it's very different to what you normally see, I'm sure your eye is drawn to it and only THEN do you read what it says.
This is why Mike's van is a good idea although I think you could achieve the same thing and have something also relevant to carpet cleaning. But it probably doesn't matter because if they are considering carpet cleaning you have them anyway.