Be organized.
Prices on everything. I lot of times I will just walk away if the comics aren't priced, where I'm forced to ask the dealer what the price is.
Don't put so many comics in the boxes that I can't safely browse through them without bending your books, or without pulling out 10 comics and placing them to the side first.
Have a clean area. I may just walk on past if the boxes are so dirty that it looks like mold might be growing on them, or if they are wet because you let them get rained on while carrying them into the show (yes I have seen this before).
Take a shower, use deodorant, and bring breath mints. Bad smells may drive away customers.
Be willing to negotiate on price, especially on a bulk deal. If your prices are firm, 9 times out of 10 I will walk away out of general principle.