At a previous house I was renting with friends we shared a 10x24-ish garage. I had my car in there for a winter while I rebuilt the rear suspension and it was challenging, because we also stored 4 or 5 spare set of wheels (winter/summer tires), a couple bicycles, camping gear, lawn stuff, skiis/snowboards, and all the other junk 3 24-27 year old guys and their girlfriends accumulate. Add in the room you need for parts as they come off the car (bumper, subframe, exhaust...) and things get cozy pretty quick.
The best thing we did was add a full loft and a winch to haul stuff up there. Anything that wasn't frequent use got put into storage up top. The other thing I did for working on the car was put it on dollies, so that I was able to have good access to whichever side I was working on. I also store things under the car whenever I can (tool kits, parts, etc.
The most important thing is simply keeping the clutter from taking over. When you've put in 5 or 6 hours lying under a car replacing metal you aren't inclined to do a nice clean up job, but after a few days like that you start losing more time working around the mess than you would just cleaning it. I know this was hard for me. More parts to work on = less space available when you need more of it. This start getting put in odd places, lost, hidden...