Thought I might take the chance to give back, after receiving tons of great advice on this forum already.
I'm opening myself up to embarrassment as this may have been covered already, and I'm sure there are better solutions out there.
I was having a nightmare removing valve springs the other night, attempting to use a large engine-style spring compressor. Due to the tight fit on my CB500, this proved very difficult and it took me literally hours to do this.
Reinstalling was even worse so I turned to the web and found a lot of proposed solutions, ranging from using your bare hands, to channel-locks, to specialist tools, and finally a C-clamp along with an 02 sensor socket (the type in the pic below with a slice removed from the side).
The C-clamp was a bit awkward to work with solo, and the 02 sensor didn't allow for much access.
Eventually I bought a slide clamp (see pic) from Home Depot for about $14, and simply cut a piece of 3/4" PVC pipe I had laying around, in the same style as the 02 socket, but with a much larger opening. The slide clamp had rubber contacts, so there wasn't the slipping I struggled with in the C-clamp, and I could compress it very easily. The PVC allowed plenty of space to insert and manipulate the valve stem 'keepers' with tweezers and a screwdriver. Finally, I discovered the best way to get both 'keepers' to fall into place was to compress the spring quite far and jiggle the slide clamp all around until they sat down. Then just release and 'click', all done!
All told it took less than an hour to put all eight back in - not quick vs jobs I've done on big V8s but I was happy considering the circumstances!