Freelancing is unpredictable. Some weeks you'll rush around working on loads of different projects. Then there are the times when your freelance work dries up and you think: 'Oh, my inbox is kind of quiet today!'. But there's no need to panic: you actually need those quiet periods.
Create an illusion
During such times, it's really a question of creating an illusion - not just for your clients, but for yourself. The illusion of 'busy-ness' is really a very vital thing. Every day needs to feel like a working day.
Personally, I quite like it when things go quiet. It gives me time to think and reflect and do some personal work. Sometimes I'll find I become distracted by working on a large project and I'll run out of time to spend on other things that I'm also interested in doing.
I try to spend as much time on personal projects as I do on paid work because those things feed into each other. Even if something isn't of any commercial use immediately, it might spark an idea for something else at a later date.
Think up new ideas
New ideas don't tend to present themselves when I'm just sitting around; they inevitably come when I'm busy working on commissioned projects and don't have the time to follow them up. I'll scribble them down on the edges of pages in my sketchbook and come back to them at a later date.
I used to be quite lazy about writing things down, but now I'm really strict with myself. I make a note of every single idea I think of, even if I've dreamed up something totally ridiculous. I don't want to look back and think: 'Damn, that could have been my masterpiece, whatever it was...'
During quieter times, it's also really useful to have some kind of network, even if it's not a formalised group - just a bunch of people you can rely on so you can send them your crap ideas and they can say: "That's awful, what were you thinking?"