Help setting up an affiliate program for services?

As the title suggests, I'm looking for help in setting up an affiliate program for certain SEO related services I offer. I've been doing some research, but still am not sure of the best way to proceed. Maybe you all have some advice?

The process goes something like this...
- Client places an order by submitting required information about their site.
- I review the information first and then invoice via PayPal.
(I have chosen to manually review the information first before receiving payment to ensure I feel comfortable accepting the order)
- Client pays and I complete the work in the allotted time.

So as you can see this is not a digital product that could pay out commissions automatically. I'm comfortable with that. I'm just unsure of how I can setup an affiliate program for the service business model I outlined above. To this point I've taken on a few affiliates on more of an honest-policy model...If the client lists a referrer I kick back a certain % of the sale price. This does not seem scalable to me though, which is why I'm here. Enlighten me friends.

Thanks,
Derek