I think the principles here can apply to any business...
You need to identify your target market. Otherwise you will waste energy (and maybe money) advertising here there and everwhere but not getting much interest. Also, getting business is about putting in the legwork not just winging out some advertising and waiting for it to happen.
I don't know much about the locksmith market partuicularly in USA. But I guess home owners are an obvious market, presumably local ones. In the UK you can put business cards in shop windows or local DIY stores with maybe a small fee. Advertising on locally focused directory internet sites may also be useful. As a home owner I'd probably head for the internet first I agree with the previous point raised about search engines etc. In the UK there are also many trade recommendation sites where other customers get to write reviews. In otherwords you get business by reputation.
I'm guessing the real and regular money is in B2B. So businesses such as building firms, maintenance firms, letting agents and landlords I guess would be good markets. Also maybe local government, schools, hospitals etc that have large premises to maintain. They are likely to already have contacts so they may not be actively searching for a new locksmith. The trick is to muscle in on business by targeting them directly i.e. calling and explaining why your service is so much better than anyone else's. Don't really on emails and letters alone. Verbal, preferably face-to-face contact is far more likely to get someone interest than junk mail. You may need to undercut or provide some value added service to to get a contract but at least you get in the door (excuse the pun). Also follow up with professional flyers or emails reminding them of how good and professional you are. Looking professional and providing added value is the key to B2B imho. Don't be put off by rejection.
Talking of professionalism I don't think I'd want some scruffy guy in t-shirt and jeans turning up to do my locks - doesn't fill me with confidence. Assuming you have a van then I would think decals for your van would be quite cheap and also make you look prefessional. Get some tidy overalls and sew on your logo (or a badge may be cheaper). Print some business cards - always always have some business cards with you even when not working. Have some flyers (& labels as mentioned by kath) handy when on business to hand out or leave anywhere when necessary/appropriate. Have a professional looking website (which anyone can do these days) and put the link on your cards/flyers/emails.
hth
Chris