Again taking from retail rewards programs are really sticky.
Since you might already be storing waiver information on the computer you can tie it in with that. If not you should be doing waiver information on the computer with the option to either opt in or out of promotions (that's how most fields around here do it).
Every 1000 paintballs and you get a bag of 100 on your next visit. The 5th rental is free, etc. Expires in 1 year and they get an email reminder.
Also flat out earning rewards dollars for every dollar spent at the field (entry, paint, other pro shop purchases) could help your pro shop compete with online retailers if you have an extensive pro shop.