Hi Karen

I think a referral incentive can be a great way to encourage clients to send their colleagues and contacts your way.
In the beginning of my practice I didn't send more than a sincere 'thank you' and maybe a special gift for each of the many referrals I received and that was fine - but once I started getting bigger contracts as a result of those referrals I really wanted to let my clients know that I was grateful not only for the kind, positive words that sent the referral my way but for the added business. I now send 10% of the new client's first [paid] invoice to the referring client as a thank you.
While I don't provide ongoing fees for referrals I know there are some virtual assistants who do. I'm not sure if contracts are put in place for that kind of thing or not. I'm guessing not, but I'll be curious to learn if anyone actually does set it up that way.
However you set it up make sure you do so in a way that truly serves
your business goals and isn't going to be a drain on your time or finances.