Great idea for a thread. I'll bet this one's going to be around a while
Some of my tricks:
Locking medical forceps are the shit. They can be used to assemble small parts, fish wires, hold parts, clamp off hoses, etc... They basically act as a third set of hands, I keep bent ones, straight ones, and every size up to 10". I've circumvented alot of disassembly steps by stealthing into a tight spot with forceps and getting the job done. I keep them clamped on the lid of my toolbox so they're always close at hand. The spring loaded squeeze clamps available at any Home Depot work well too. Keep a selection of sizes up to 4" or so. You'll wonder how you ever got along without them.
Rare earth magnets in a bunch of different sizes really help out around the shop. Get several of them in sizes from a tic-tac on up to a few inches across. The round ones are handy because you can stack them up for more holding power yet still have a narrow profile to reach tight spots. If you need to fish a part out of your bikes innards, just put a few tic-tac sized ones in a condom or other flexible holder like a drinking straw with the end taped off and go bobbing for lost parts. The bigger ones can be used to hold parts in place while you futz aroud trying to get some impossible nut and bolt attached. If you've been cutting metal, stick one in an old sock and drag it around the vice and workbench. Hold the sock with all the shavings now stuck to it over a trash can and just reach in and pull the magnet out. Presto! The shavings fall neatly into the trash can
This trick works well on welding/cutting slag too. You can also check the amount of metal after an oil change by putting a magnet in a zip-loc bag, swish it around in the old oil and see what it brings up. Could give you an early warning if a problem is coming up.
As was stated in another thread around here, Automatic transmission fluid is a great penetrating lubricant and solvent. It'll free up most any stuck or gummed up assembly. I keep a squirt can of it around all the time.
I'll think up more for sure