I don't call them samples, I call them 'travel soaps'. I sell them and they are popular with non corporate travelers. Have you seen the choices available for travel soaps in drug stores? dismal.
When someone makes a purchase above a certain amount (you determine that for yourselves, for me it is $20) I give them a complimentary travel soap - and tell them it is a chance to try one of my .........fill in the blank soaps. They already bought something so they are now a customer. You'd be surprised how well this goes. Everybody likes free stuff! (even Oprah I hear).
This is not a hard fast rule. When someone comes by my booth and I am sold out of their favorite soap, I'll just give them a travel sized one to tidy them over, if I have them of course. It might be a person who just buys one soap, but they have been doing so for a while. 'I will have them again next week, here is this travel one in the mean time' Again, they really appreciate this and they do come back when their favorite soap is cured!
Once I gave soap samples as halloween treats (clearly marked as soap do not eat! I even asked the kids to read that part to me before giving it to them). Nothing ever came out of this free soap hand out. I will not do this again. However, samples for regular customers are a very good idea.