I rarely find this an issue. I've found I'm more likely to be waiting for a client to be getting back with some sort of feedback, and therefore using my time to be working on another project.

This may be down to handling client expectations at an early stage. If you make it clear that as a freelancer you will be working for several people at any one time, and give them a time estimate for when progress will be made, I've found most clients are fine.

This does mean I have to make sure I'm progressing enough on each project to keep each individual client happy, but I've found for a lot of people, as long as they see some progress and you're keeping up communication with them - which can be as simple as a 'here's a quick update' e-mail - then they're happy.

If you get a client who wants you to work solely on their project, this can be more of a problem. As I've been freelancing a while I'm in a position to say I can't work that way, and know that I'll find other work if they then do not want me to work for them. When I was starting out and work was thin on the ground, I couldn't be so picky. Then again, I didn't have as much work to juggle either.