How to get customers.
Talk to folks. Will the show promote you?
I'm not a fan of the no fee model. I want at least something up front even if it is bundled into a package.
On site proofing can be awesome if you have time to do it, but if things get busy it's a pain. More than an experienced handler, I'd say you need someone to run your proofing/viewing if you do it on site.
Are taking appointments only or walk-ins?
At my fundraisers, I do 4-6 sessions per hour during the busies times. Proofing is done during the slow periods or after the show on the website. I have had good luck selling pre-paid packages with no viewing at all, but this is at shows where the folks have seen me for a few times now.