It's always a good idea to have a reciept book to write up hand-written receipts, even when you have a printer. Never know when the printer will go down.
If you can only take cash, put a printed sign up stating that.
Doublecheck the sales tax.
Have double the number of business cards you think you'll need, everyone always seems to run out at busy shows.
Keep the animals and the enclosures CLEAN. Nothing shows worse than a dirty container.