Congrats on being in a position to sell. Just spitballing here.
An extensive and on-going client list is a major asset. That and the turnkey nature of the deal warrant a premium. How much of a brand have you built? Do clients recognize "you" are the brand, or does the company really have its own brand and culture? Meaning, with you out of the picture, how would the company fare in regards to recognition, repeat clients, service etc?
Have there been any other sales of your type of service-business recently in your area or neighboring communities? Identifying comparables could get you in the general ball field, particularly if sales figures or net incomes are known for the comparables.
Something else to consider is how your prospective buyer will fund the purchase. If going with a traditional bank loan, the lender will likely have its own idea of valuation as far as assets to loan against and forecasted cash flow. Getting in touch with your banker could give you further insight on asking price.
There's always third-party valuation. Your CPA analyzes your business and puts it in context versus the industry, will probably run some discounted cash flow analyses as well, pay back periods, etc. The valuation will also depend on the certainty of the cash flows.
Good ol' fashioned negotiation? :)