Having spent the day affixing all manner of things to our walls, I figured it might be worth jotting down some tips and hard-learned lessons:

• Be careful when measuring with a tape measure; it's best to measure from the 10cm line and then just subtract the difference, as the end isn't always that accurate (or better yet, buy a proper ruler!);
• It's fine to tap a wall plug into a hole as much as you need to make the screw fit, particularly in plaster (screws don't seem to grip plaster too well, so the last bit isn't really that important);
• A reversed drill bit or arbitrary steel rod are useful (plus a hammer) for the above tapping;
• Without fail, our wall plugs ask for holes 1mm too large. If it says 7mm, use a 6mm drill bit; if it says 10mm, use 9mm, etc.
• Screw thickness does more for strength than length; length helps it grip more, though, and a combination of both is always preferable;
• If a screw is twisting at the end of insertion, it probably means the wall plug is too small; try the next size up;
• Screw holes being too long isn't much of an issue, really.
  

Probably nothing too spectacularly novel here, but potentially useful nonetheless.