I have recommend a friend vouchers and they do work for me.
You can operate this scheme by actually giving very little away! My vouchers give the new client 10% off a full set of nails (or Spa mani), and the friend who recommends them gets a free Creative enamel.
Firstly, the new client: Supposing they have a full set of nails at £30, with the 10% off voucher they pay £27, BUT they do need the solar oil pinkie at £3, so they still pay £30. Basically you have got the full treatment price and have given them a free S.O pinkie so you have spent a pound to get that new client in. Also, remember that you will issue this client with a voucher so they also have the potential of getting a free bottle of enamel from you.
Then, your usual client who recommended them. To me there;s no point giving them money off. They are a client, they were going to spend that money with you anyway. Rather than saying that their next treatment will be a couple of quid cheaper, to say that they will get a FREE gift is much more enticing. Also, that free enamel is WORTH £10 to the client but has COST you about £4, so again you are giving away very little. The whole transaction between new and existing client has cost you about a fiver, but they both feel like they've gained more than that.