Hi Janice

I don't really have an official 'system' in place although we always, always thank clients in one way or another for referrals - either financially (a relative discount on services/invoices) or via tangible goods and gifts. It just depends on the circumstances. I do know there are quite a few VAs here who do have actual programs in place so surely others will pitch in as well.
In the meantime, here are a few other threads you might find useful/interesting
Client Referrals
Referral "gifts"?
And an archived podcast called
10 steps to Building a Winning Referral Engine (absolutely worth listening to!)