I’m reluctant to reply because this subject has an enormous set of answers, but each of them typically apply in very specific circumstances. (I used to teach this class, and I was always impressed at how my students discovered new and successful methods.) What follows is necessarily very generalized.
It’s important to be able to explain, very succinctly, exactly what your expertise is, which might be stated as “what problem can you solve.” In that explanation it’s critical to also state a value proposition. Example: “We engineer, install, and maintain very high reliability and high availability wireless networks for small to mid-size businesses serving the East Tenchurch, Nebraska area.”
You may do many other things very well, but each of those has to be considered a separate business for marketing purposes.
Once you have your value statement, make sure it is clearly spelled out in everything you do, including your web site, your business cards, and your stationery. In a business setting, always introduce yourself with that phrase.
If you provide services to a specific geographic area, consider renting a mailing list and sending postcards. Hint: on the postcard be sure to state, “Save this postcard so you can find us when the need arises, because it will.”
You can never network enough! Join every group you can identify that has regular meetings, show up, be open and friendly, and sincerely inquire of each person you meet to find out what they do, where they do it, and what’s their most pressing concern (not problem!). Make sure you give them a business card, and remember to say, “Save this card; someday someone you know will need our services and they will be grateful for your help.”
Have a separate cell phone for your business and answer it whenever it rings, including in the middle of the night. Yes, you will have to fend off a bazillion spam calls.
Make sure to remind every one of your customers, repeatedly, that you are always looking for new clients and will be extremely grateful for any referrals. Do not promise money! And make sure they have one of your “save this card” business cards.
Do all of the things in the previous response!
Write back and tell us just a little more about what kind of business you run. Better yet, tell us what your value statement is.