You have to offer them something they feel they have to have.
I just made some postcards from vistaprint offering a free eyebrow wax treatment. I could have added with a haircut, which I didn't, b/c I want the good word to spread about me, and the opportunity to offer other services while they get their free service.
I will be handing them out personally to people (business owners, employees) the type of clientele I want to have.
I find offering a percentage off doesn't give the customer a hard monetary value in their head. Most people can't do that kind of math in their head.?Something like, 5 quid off your first haircut?
If you have a website, you can have a code put on the fliers they could punch in so you can track it.
If want, try offering something that doesn't cost you anything but your time and a little bit of product, free buff and polish, fringe trim, whatever you want.
Don't over discount, then people think you are desperate, and not very good.