Sure, here are a couple ideas.
1) Goal based rewards- Reward your client with some discount on your services and/or some freebie like photo adjustment, or additional shots, etc. Of course, you need to have a website for this purpose with online-ordering functionality as all tracking works online.
2) Referral Contest or Sweepstakes: Reward more active referrers or define winner by random. In these campaign types it works better to offer more valuable prizes, for example, free photo-session or something like that- invite 3 friends to be included in our sweepstakes for a free photo shoot, etc.
Also, you can offer the friends being referred a coupon for your services at the same time automatically. Feel free to check out our blog for more reward ideas at InviteBox.com.