This is a common question often asked here on VAF.
Many VAs have an ongoing referral program. My own is 10% off the next invoice if the referrer is my client. Or, if they are not my client I send 10% in the form of a gift card with a handwritten note thanking them for the referral.
If you use the search feature here on VAF in the upper right corner, you can search for other responses. Here's one I answered
where I explain never to give free hours in return for a referral. That could be a problem later on.
So glad to hear you are picking up clients through referrals. It is really the best way to grow a business.