You'll need a dye-sub printer ideally (quick, no messing about, just press the button and a few seconds later out pops a print), a way of your customers selecting an image and a way of them paying.
I did my first print-on-site event a few weeks back. I shot with a custom white balance, small jpeg to one card, raw to the other (just in case someone wanted to order a huge print). Once I'd finished the shots the guests went to lunch. This gave me chance to download the shots to my Macbook, do a quick selection in Capture One to avoid the blinkers, then create a catalogue in Media Pro and print contact sheets. Under each image was a file number. I laid the contact sheets out on a table for customers to choose.
I asked the customers to give me the last three digits of the file they wanted, had a custom print profile ready to go, popped a 6x4 out, I had my dad assisting, he popped them into a strut mount, all ready with one of my business card stickers on the back, then into an envelope.
When I got home I also uploaded the files to a dedicated area of my site so anyone who missed them could order online. I also made a small size digital download available for a nominal fee for Facebook etc. The link to this was emailed out by the organisers of the event to the attendees.
I hope this helps give you an idea, feel free to shout up if I can help any more, although there will be photographers on here with infinitely more experience of event photography than me.
