I'm a shoe tester. I got in and have stayed in the program (around 8 or 10 trials so far) because:
1. I wear size 10
2. I run the correct mileage for the program (30 - 60 mpw)
3. I fill out my logs faithfully.
I applied at a booth at a marathon expo. I'm a supinator so I can only test shoes that are neutral and flexible without getting myself hurt. If after a week or two my IT band or iliopsoas is screaming at me then I usually contact the program lead and let them know that I can't wear that shoe any longer because it has too much motion control. They are very cool with that and say thanks for trying.
You have to be pretty selfless in some ways because if you take on a shoe for testing it is an 8 week commitment. If you don't really like the shoe you still need to put on enough mileage in it to give them the wear data they need.