For new customer bids, I always have handy a stack of the ISA brochures about how to hire an arborist, why tree topping is bad, etc. for educational purposes. I bring all the usual paperwork and company brochures, etc. plus I always have my full climbing gear in the job box in my truck, as well as a hayate pole saw for pulling hangers, etc.
I have just bought a laptop in the last month, forcing myself not to be such a chicken about technology. It has my quickbooks program loaded on it, so I have all my customer history with me at all times. This is so nice for when they want to know what we did last time, when was it, how much was it, which arborists did the work, etc. It also has all my financials on it, which may be a bit of a security issue, I guess, but I have an open book policy with my employees, and when they want to know how it is going, the reports are awesome in quickbooks.
I keep a stick of deodorant, a clean shirt, a sweat towel and toothbrush so I don't look or smell like a total slob when I have left the crew to go give bids. (As rare as that is anymore!)