As welll as the posts already made (signwriting on van, on-site signs), word of mouth is good, as it implies happy customers. Keeping customers happy is a job in itself, and probably could do with a book.
Get in touch with local architects who may be commissioned for extensions, dormers etc (planning regs have made architects a necessary evil for all but the most simple project). Many local authorities have a commitment towards local businesses, and whilst they may have few obvious pieces of work (though an angle might be remedial works after vandalism), once you are "in" a lot of work will by pass the initial hassle of full competive tender and simply to put out to competitive quote. Registered social landlords (aka Housing Associations) are a source of repeat work as tenants have a nasty habit of trashing homes, (but note RSL.s can be notorious slow payers whereas most local authorities are very good.
Mailshots are the like are for people who think they want to build a business but are afraid to ask for it in person.