This. I believe Tomosy also suggests figuring out what each part does and understanding how it works with the parts around it before removing it. That can help with reassembly, since you don't just want to put it back together, but put it back together working.
^^^This too! Understanding how everything slots together is very important and can make reassembling things so much easier.
The slightly odd looking cog just visible in these two pictures for instance:
It has a funny cut-out type shape to it, like a chunk is missing. Weird... Why is it like that?
It took a moment to figure out what it's purpose is, besides linking to another two cogs with a tensioning spring at the end of the chain. It controls the mirror! It flips the mirror up just before the opening shutter fires, then spins and releases it as soon as the closing blade has shut. A lever with a roller on the end is actuated when the shutter fires by rolling over the top of it, but when you re-cock the shutter, it turns in the opposite direction so the roller goes underneath it. Clever!
It's also good to bear in mind the following when re-assembling:
Whatever camera (or other mechanical or electronic device) you dismantle was designed by someone who probably knows far more than you do about camera (or whatever) design. Each part, screw, washer and/or shim has been thought about and built into that design. And designing complex mechanisms is hard work; no-one adds seemingly pointless bits for fun, everything on that mechanism is there for a reason, even if it's reason isn't readily apparent. So... Make sure you put everything back exactly in the reverse order that it came off in. If you end up with no screws or washers left over after re-assembling, you win!