Keeping good notes helps me switch between projects quickly. I jot down ideas as they come to me, and also write myself a quick note whenever I get pulled away from one project: for example, "left off at line xxx in file xx.pl, need to finish the data checking code for types y and z".

I keep a text file open in emacs for each large project that I'm currently working on. Any time I have an idea, I switch buffers and jot it down. At the top of the file are notes about the details of what I'm currently working on. Then I have another file with notes about shorter projects and other things I need to do. These files are mostly ordered top to bottom by priority. At the top of my main file is a section that I like to clear by the end of the day. I don't just keep programming notes in here -- I also include notes about emails and phone calls that I need to respond to, etc.

I'm not suggesting you adopt my system (maybe you're a vi person :), but having a good organizational system like this will really help you alternate between many projects.