How do you manage your side-projects? Do you develop different projects at the same time or you focus to one of them until it’s up and running then mess with the other?
The one thing I try not to do is burn out on one project, when I feel that happening I shift to something completely different or just go read a book for a couple of days.
Right now i'm working off and on on 3 web projects, and a couple of c/c++ tutorials, and i can forget about by own self-imposed timelines and deadlines at the drop of a hat anyway, but I find that it's a lot easier to get somewhere on anything by just focusing on that for a while.
In terms of efficiency, concentrating on one task at a time is best, but as people, we also get bored, and look for amusement/distractions. Other projects are a useful way to cater to this need.
One major/difficult project, and a handful of trivial/noncritical/short-term projects are a good combination. Don't take on more than one major project at a time.