Coffee and loss of sleep!
If you are making a living as a freelancer, it can often be "feast or famine". Sometimes there is not enough work coming in, sometimes there is too much!
Above all else, make sure your customers get what they want, in a timely manner! Have good communication and be honest. If your overloaded and something is going to take you longer then expected, let them know ahead of time. Keep your customer satisfaction high, and you will succeed.
Find other freelancer. Get to know them. Know each others skill sets, strengths, weaknesses. This way, if your overloaded, you can sub-freelance to get the jobs done!