Print some vouchers off on your computer. Space for name and address. Everytime you do a job fill the details in on the vouchers for that customer then hand them to them, 5-10, at the end of the job. Tell them that for every 3rd voucher you recieve back they'll get £20.
I did this for 2 years and it cost me £200. It amazing how many 1's and 2's you get back which cost you nothing.
Lots of customers say, "thats a fantastic job, I'll pass your name on to my friend and family" they never do but offering them an incentive then a lot will.