I'm retired now and spend some of my time helping a local IA on annuals. Several of the airplanes are Mooneys, two are TLS. It can be a real shit show.
Most owners are not hands on with their own airplane between annuals. Some don't even clean the windshield.
A short list of things owners should do before sending their airplane for annual (from my viewpoint):
Clean the lead and oil off the belly and wash the airplane exterior including the wheels, brakes, and wheel wells.
Clean all the bullshit out of your airplane interior. Empty bottles, empty and full oil bottles, snack wrappers, crumbs, old flight notes. Don't leave your headsets in there. Remove anything in the way of removing the seats.
Make sure your airplane has all the placards listed in the POH - they're required. No unmarked switches.
Make sure your POH is complete and not falling apart, the weight and balance document is there, and any additional required accessory operation manuals (GPS, autopilot, engine monitor).
Keep you AC records organized including the current list of complied with ADs and reoccurring ADs. Keep the continued airworthiness documents in a binder or folder - these are part of the aircraft maintenance manual.
Generally, its a good idea to keep an inventory of the date and hours for the last replacement certain items:
Magnetos, sparkplugs, brakes, battery, tires, avionics filter. Add turbocharger and tailpipe clamps for those turbo models.
If you defer having things fixed, by all means expect the annual to take longer if you want them fixed then.
Nobody wants to do an annual on junky, poorly cared for airplanes. They are viewed as a risk by the IA's.