The come-and-go of work is one of the most difficult things about freelancing for many writers. It's often referred to as "feast or famine," and although I've never experienced it quite to that extreme, there's definitely some truth to that cliché.
I typically find that my busiest period is during the summer, and my slowest period is in December and January. Unfortunately, that feels opposite of what you would want: It would make sense to me to have less work during the summer, when the weather lures me outside, and more work around the holidays to provide extra money for gifts and other celebratory expenses.
Part of surviving slow periods is being able to lessen the impact, either by planning ahead or by having an alternate source of income. I'm admittedly not very good at reserving money for slower periods, but luckily I do have an alternate source of income: babysitting.
I have a long history of working with children: I worked as a preschool and after school program teacher for about six years during high school, college, and the years I took off in between. Midway through college I switched over to self-employement (though I wasn't savvy enough to call it that then) and worked as a part-time nanny and babysitter until graduation. After I graduated, I immediately got a full time job as a technical writer, but I maintained relationships with one or two of the families I had been babysitting for, which also helped to ease the transition when I quit work to freelance full time in 2005.
Fortunately for me, I get the most demand for my services as a babysitter right when I have the least demand for my services as a writer: around the holidays, when there are lots of parties to go to. It's perhaps not as esteemed as my other job, but it helps to smooth out the unpredictable income — not to mention it's pretty special to watch these kids grow up!
How do you deal with the "feast or famine" phenomenon of being a freelance writer?