We are a software development company with only 8 employees but all of us are seniors in different fields. We have developed 2 products from scratch for a big international company with over 35k employee (6-12 months developments). One of them is an app which is mandatory to use for all of the 35k employees. We passed all request from integration with Active Directory, SAP, GDPR, Russian legislation, Penetration tests and a lot of paperwork which are highly demanded if you are working with an international company.
Below are my suggested methods. There are many more but I don't want this answer to be too long.
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.
3. Publish as much content as possible on your website - 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 focused 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, including programming companies. These companies give me a small percentage of the profits they make from my referrals (of course I only refer to companies which I know and trust as my reputation is worth more than the referral fee).
I've successfully helped over 300 entrepreneurs and will be happy to help you.