Typically you'll want to send a demand letter that outlines the amount you are owed, the terms of the agreement under which the fees are owed, and your ability to pursue legal remedies if the amounts are not paid. Often these letters have more force when they are written and sent by a lawyer.
Do you have a written agreement with these customers? Are you willing to accept less than the full amount? Are you willing to allow the customer to pay according to a payment schedule, i.e. not a lump sum payment?
These are some of the questions you should have in mind when you decide how to pursue the collection. It'd be ideal to sit down with an attorney to discuss more of the details surrounding your situation. You can contact our Tacoma office to schedule a free consultation if you'd like, http://tacoma.invigorlaw.com/
You don't say what kind of business you have and what types of services products you have not been paid for providing.
A mechanic's lien is only for improvement to real property by subcontractors and suppliers. If you are one of these, then you have to worry about the other very strict rules on filing liens. It would take a book to tell you everything.
My recommendation is to check Avvo for a local attorney that prepares and files mechanic's liens, if you are eligible, or a business or collections attorney if you are not. Do so sooner rather than later, mechanic's liens have to be filed in a very short period of time.