The best option is a mail store which provides you with an actual street address.
Yes, I and others have been strongly recommending to Google Maps engineers that they come up with an option for contractors and other street-address-less businesses. Google Maps encourages users to go to them for what were traditional yellow pages or internet yellow pages style searches. But broad categories of electricians, plumbers, roofing contractors, painters and more need an equitable provision for the companies/individuals who do not have actual street addresses.
The introduction of PlaceRank in the influencing of rankings just further emphasizes the disparity.
Map really needs a method for displaying some sort of graphic indicator for businesses which do not offer a physical location.
This is one area where internet yellow pages - IYPs - (which developed out of printed directory companies which have been providing business listings for nearly 100 years) have been superior to Maps.
Since some of Google Maps' data partners (such as IYPs) have addressless listings in their databases, you could also just register your business with one of them and wait for that listing to become reflected in Google Maps. It could take a while before the update were to go through, though.