I'm often working on multiple projects and what works for me is the following:
Clear understanding by those involved on each of those projects (that aren't single-person ones) that on X day I'm only working on the ABC project and that any questions will be answered on the next day for that project.
I don't context switch back and forth during the day. I work on one thing and get that to a point where I can change to another project. I find too much flip-flopping between projects causes me to do less than 'my best' on them.
I utilize a prioritized to-do list of things that need to be done. Even if I'm on a single project during a certain period, I still utilize this. Then I don't have to try to hold that stuff in my brain. What I write down, I don't have to try to remember.
I keep a folder (literally a paper folder) for each project and an electronic folder for each of the projects I work on. Everything goes in the folder when I'm done working on it. All my notes are kept separate and then I'm not flipping through a notebook with multiple notes for multiple projects interspersed. I have an electronic calendar for each of my projects that is rolled up into a master one so I can pick the granularity of my "let me check my calendar" situations.
Everyone has to be "on board" with my multi-project work. Those that I report to need to be aware of it (and reminded of it, as necessary) so that they don't try to get 100% of my time whenever they want it.
I have to remain a little flexible on the off chance a fire drill pops up. But it needs to be a real fire drill. While I'm not that into the "self help" books, I do like the Covey idea about determining whether things are important or not important, urgent or not urgent.
If needed, I turn down or postpone work that I simply can't take on. I'm one guy. I don't have a problem saying 'no' but I only say it when I mean it. Make sure you aren't overcommitting yourself. The last thing you want to do is over-commit and under-deliver. It needs to be the other way around.
Anyhow, that's how I do things and it generally works well for me.
Best of luck!