You need to expand your business. Yes, sometimes you are overloaded but other times business is slow. You need to have clients requests at 130%-200% of your capacity so you can stay fully booked. When you have this level of business you can refer or refuse some work to fit your schedule. Develop relationships so you can off-load this excess. It also provides you an opportunity to absorb some people to expand whenever you decide the time is right.
To get there you need to work on your business. Thinking strategically and network a large part of the time. If you are spending a lot of time on back office and operations you reduce your ability to pick up new work. Consider outsourcing some of the back office work. Xilium has you covered with college educated, professional virtual assistants.