1. If your preflight check is interrupted start again. Carrying on after an interruption leaves you more likely to have missed something.
2. Pledge household cleaner is great for getting dead bugs off leading edges etc. Spray the mucky bits after shutting down and then by the time you've sorted out the accumulated junk in the cockpit the splattered remains will have soaked up the polish and will wipe off easily. Sparkle is the best perspex cleaner I've found.
3. After checking the fuel for water, don't chuck it on the ground, pour it over the nearest tyre. 100LL evaporates too quickly to damage the rubber and will leave the water droplets on the tyre if any water is present. Much easier to spot than doing it on oily, dirty tarmac.
4. Always leave metal props vertical to prevent water accumulating in the spinner especially in winter when the water may freeze before startup and unbalance the prop / spinner.
5. Always leave wooden props horizontal so that any moisture doesn't soak into the lower blade and throw it out of balance. It would have to be stationary for a long period for this too happen. If a spinner is fitted, rotate the prop by a few revs (mags off!!) to see if any water has gathered a la 4. above.
6. Get a cover for your windows especially the windscreen. Saves a load of polishing and scratches and stops the blazing sun (UK, yeah right) cooking your (expensive and delicate) radios etc
7. Polish minor scratches out of perspex using metal polish such as Rolite. A bit of effort involved but cheaper than new glazing
Works for sunglasses too.
8. If the aircraft you are preflighting has rippled panels on it, don't fly it, call an engineer.
9. Grap a wingtip and move it smoothly and fairly firmly up, down, fore and aft. Sounds silly but if a clot before you has dinged a wing and not reported it, you might be the unwitting victim. Remember the Robin crash a few years ago after a guy hit a haybale on landing? Next person who flew it had the wing fold on them.
10. Always check for full, free and IN THE CORRECT SENSE control surface movement after any maintenance work involving controls has been done. People have accidentally connected ailerons up the wrong way around since Pontious was a pilot.
11. Don't forget to have fun