How do you say thank you?

I just want to find out how people say thank you to the person who give them a referral. I just send a thank you email. I do not get many referrals.

I had 2 referrals recently. These are not from a client. One of them has become a client.

Do other accountants say thank you in one or more of the following ways:

1. Cash. Is this a % of the fee?
2. Gift - Vouchers
3. Thank you note (email)
4. Do nothing

I want to thank the person who gave me the referral. I am thinking is a thank you note enough? I am sure I will get  good ideas from reading what other accountants do.

Thanks