Right now I have several more clients than I'd really like to have. They are mostly good clients overall, and I enjoy the work I do for them. However, work is never 100% steady with any of them; any given client might give me steady work for several weeks, and then not have anything for me to do for a few weeks after that. All of my contracts are hourly, no retainer, so if I don't have anything to do for anyone on a given day, I don't make any income that day.
Most of the time, things balance out. Even if I don't hear from a few of my clients in a given week, I'll have enough work to do for the remaining clients that I can get in a normal 40 hours for the week. Of course, things don't always work out so neatly. If everyone sends me work at once, I can barely keep up. Conversely, sometimes all of them seem to go on vacation on the same week and I'm left sitting on my hands.
I don't feel comfortable taking on any more clients because I'd be completely overloaded if everyone sent me work at once. I also don't want to start dismissing clients with whom I've had good long-term relationships just because there is some downtime. Nobody is ever willing to pay retainer fees, so that idea is out as well.
Are there any good strategies to mitigate downtime or loss of income without adding more and more clients?