Hi,
Good question. There are many methods, but I don't want this answer to be too long so here are a few of the main ones:.
1. Ask your existing client/s for referrals. If they were happy with the work that you did, they should be happy to do so. If you feel the need, offer them a 10%/$X discount on the next project for every client they refer. This is 'win-win-win' situation because you get more clients, they get a discount, and they use your services again.
2. Ask your existing clients for a written recommendation which you can publish on your website + permission to use their logo on your website. You can offer them a link to their website which will be good for both of your SEO rankings - so another win-win.
3. Publish as much content as possible on your website. This can be about the projects you've done, potential projects, your fields of expertise (obviously you need to have a professional and trustworthy looking website). Each article/post should be about 1.5 pages (Microsoft Word). Make sure the content is adapted to your customers (so probably not too technical).
4. Create a free 'get a quote tool' - many customers check online to get an estimate of how much the project will cost. If you have an automatic online tool, this can attract a lot of customers. I have personally used such a tool and I was very impressed with the company's website and free tool.
5. Partner with organizations/companies that work with your target clients - for example: I work with lots of entrepreneurs and many of them ask me for referrals to various types of service providers. These companies give me a small percentage from my referrals (of course I only refer to companies which I know and trust as my reputation is worth more than the referral %).
p.s.: LinkedIn can be an incredible tool if used correctly.
I've successfully helped over 300 entrepreneurs and will be happy to help you.
Good luck