I do free 10-minute demonstrations when things are slow. (These are fully clothed, on the table or the mat. I don't like chairs.) I work in a busy acupuncture clinic, so I put up a sign in the front office and whoever happens come in that day can get free ten minute session. I've tried doing it on a regular schedule (like every Friday) but since most of the acupuncture patients are coming in on a regular weekly schedule, it would be the same people every week.
Very few people book sessions after a demo, but it puts me in their mind if they are ever looking for an MT. I'll do get referrals from people I gave free treatment to even if they've never booked. And I will always give one free ten minute treatment to anyone who is in a position to give good referrals (like a local naturopath or hairdresser) if they express any interest. My "quality of touch" is my strong point, so if I can just get my hands on a person, they'll usually give a very enthusiastic referral. I'll also give a free ten minute treatment to hesitant folks whose friend/spouse/etc is a client of mine and is trying to persuade them to book a session. That has been very successful in getting clients who come in a few times a year, but no regulars from that.
I don't see the point in giving away more than a ten minute treatment, and it is one per customer. Ten minutes is more than enough time for them to decide whether or not they like it.
Free samples are useful for products and services the customer is reluctant to take a gamble on. The target for free massage samples is people who are curious about your style of treatment, but afraid they won't like it or that it won't be worth the money. It is great if you do an obscure modality or are adding an unusual new service to your offerings.
I think "free with the purchase of X", can work provided X has a good client base. I wouldn't count on it persuading people to buy X because they are so interested in getting Y for free. However, people who are already paying for X are more likely to also pay for Y next time (if they like it) than people who hadn't previously expressed any interest in X or Y.
Offering a sample of an unusual service is also good because it makes it sound less like you are desperate for business. You can approach it with the attitude of, "Oh! You've never tried [Modality Y]? You should try it! Everyone should experience [Modality Y] at least once! Here, let me give you a little demonstration." Confident cheerful enthusiasm, not a sales pitch.
That is the angle I take. It works well for me because I offer a lot of strange adjunct therapies that people are often very curious about but unwilling to make a gamble on something they might not like. I'll even demo on someone else (like the receptionist) while they watch if it is one of the "You want to do *what* to me?" treatments like cupping or salt-moxa in the navel.
-- Joshua