Sorry for the late response. Just returned from a much-needed vacation
I've been an "Agency of One" since 2006 (with the exception of a brief return to full-time in 2011). Three things have made the biggest difference for me over the years: 1) leveraging my professional network, 2) focusing on results and 3) learning to be a more effective business owner by delivering high quality to clients without sacrificing profitability.
Before diving into any project it's crucial to make sure it's a good fit for both you and the client. For me that means having a definitive sales process and pre-qualifying clients before making any long-term commitment. It's kind of like dating
A prospective client needs to be willing to invest some time up front into a thorough discovery process so we can collaborate around what they think needs to be solved vs. what really needs to be solved (we often unearth deeper issues during the process and they get better value as a result).
It's also critical to keep your expenses down and your productivity high. I don't outsource anything unless absolutely necessary. I do have a VA for admin work and sometimes outsource technical and visual design when it's in the best interest of my clients.
For anyone wanting to start or grow an agency, I highly recommend Ugurus 10k Bootcamp. It made a huge difference for me. You can learn more about my experience in a recent podcast here: http://digitalagencyshow.com/episode11