We run a micro-business and we ask people how they heard of us (and use offer codes, but that was mentioned). People are usually happy to tell us; but then we

We run a micro-business and we ask people how they heard of us (and use offer codes, but that was mentioned). People are usually happy to tell us; but then we only use that information to run our business better, we don't sell it. Another technique is referral bonus, "recommend a friend" type offers.