i did a A5 sheet folded into 2. On the first half was the client's name and a discount of £5 off next treatment when someone they recommended came to me. the other half was an invitation to come and try a treatment and get 10% off, with my details on it. It also had the client's name on it, so when it was handed in - I knew exactly who it was off.
I gave out about 30 of them, and got about 8 people taking up the offer.
I designed them myself so could put any details on them, and did a dotted line down the middle for the client to cut it in half.