As jack has mentioned go to the end zone where the Bills will be warming up. You might have to move occasionally as the players go out of bounds sometimes. Highlights for me have been standing behind the end zone while the offensive line practices snaps and being amazed at the size of these guys. The pictures that I got including with Thurman Thomas on the Bills sideline, seeing Jim Kelly on the sideline but you wont get pics with him, he's usually doing a private escorted tour but not sure what he'll be doing now.
A feeling I'll never forget on a foggy, misty day being on the field at a Bills Jets game and hearing Phil Collins Air of Night booming from the speakers. It was kind of a surreal moment and I will do on field every chance I get.
Make sure you get there early enough to enjoy it as it is a really cool thing.