I find two channels work best for service based businesses when generating early traction - content marketing and sales. This is what I would do:
Before starting anything, I'd build buyer personas. Who are the people you want to do business with? What are their challenges, pains etc? What does your offering do to solve those pains?
Then I'd build content that will interest and attract those personas. These can be directly related to my value proposition in a way that helps them overcome those challenges, OR I'd focus on other topics that they love to consume, find top performing posts and build upon them to make them better.
I would probably have two different offers during this content funnel - an eBook and a free consultation. The eBook offers something valuable upfront and allows me to capture email addresses in order to nurture them. The free consultation is for those who want to take action. Either way, you want to guide all visitors to this offer.
Don't forget - distribution is key. Find where your personas are and distribute your content there. Same goes for influencers - build these relationships as soon as you can.
As for sales, I'd make a list of the companies I'd love to do business with, and email them with hyper-personalised, useful and short emails. The number of emails in the sequence would range from 3 to 8 depending on what I've got to say. Any more than 8 is overkill (some might say the same about 3, but my experience says otherwise).
I would work the hell out of these channels and test several approaches. The content marketing approach is building an inbound engine, which will happen overtime, and my sales system would be looking for opportunities in a proactive manner for quick traction.
Also, if you're been around for some time and already have clients, ask them for referrals. Referrals are gold.