It is a process, not an event.
First off, maintain the aircraft.
A well-maintained aircraft will always have a higher residual value than one that is not. Well-maintained in this case refers to an aircraft that is maintained beyond what the minimum regulatory standards require. Airworthiness is related to safety, not value.
Being well-maintained means more than keeping up with the required inspections and component overhauls. It means the aircraft has its equipment in functioning order, non-critical wear and tear items are taken care of, and cosmetics are recognized as important, too. There may be optional service bulletins that add to the functionality and maintainability of the aircraft. These optional service bulletin items should be chosen with maintainability and mission effectiveness in mind.
The aircraft exterior should be kept clean and polished. Interior comfort and convenience items need to be maintained in good working order and updated as required. A clean aircraft is not only more appealing, but problems and issues are spotted earlier and thus, may be easier and less costly to remedy. Paint and interior should be kept in good condition continuously, not just in the week prior to putting your aircraft up for sale.
The best ones to maintain the aircraft, if possible, are your own in-house maintenance staff. A well-trained, dedicated maintenance staff is your first and best opportunity to keep the aircraft in top condition. Your choice of a major MRO (Maintenance, Repair and Overhaul) facility is important as well. Picking an MRO is more than just going with the lowest bidder. The MRO must have the knowledge and skills to perform the required maintenance, and when necessary, troubleshoot and repair your aircraft. They must do so in a manner that inspires confidence in their work. They do this by meeting schedules, communicating regularly, and by returning your aircraft in such good condition that follow-up work is minimal, if at all required. In-house maintenance plus an MRO with a top reputation for your aircraft type is a one-two combination for maintaining your aircraft value.
Maintain the aircraft records.
Maintenance records are the health record of your aircraft. Detail beyond "Required inspection complied with" helps with maintaining your aircraft value by providing a written record of the quality of your aircraft. When I see my physician for my annual checkup, I want to hear more than "looking good, see you next year." What is my weight, blood pressure, cholesterol, etc? The aircraft maintenance records not only need to be a true and exact representation of your aircraft, they are also the written proof as to the quality of the maintenance. This is the tool to use to communicate the health of your aircraft to your maintenance staff, MRO, and any future buyer. Missing and incomplete maintenance records call into question even the basic airworthiness of an aircraft.
Be careful with upgrades.
As aircraft age and the production numbers get sufficiently high in number, there are always third party companies that seek to enhance the aircraft in terms of functionality, performance or looks. When evaluating an aftermarket upgrade for your aircraft, you need to consider the impact on the aircraft's value.
Does this upgrade represent a step forward in technology closer or equal to that of a new variant? If the current production models have a particular feature that is also offered as an upgrade to your model, then that likely adds value to your aircraft. An example might be upgrading your old avionics to the current production model's avionics suite.
Is it something that is going to be commonly accepted by most buyers? One-of-a-kind is great in art, but not with production aircraft! If your aircraft is one of a few upgraded with a particular modification, then where or who will be able to work on it? Why isn't that feature more commonly requested or installed?
While first and foremost, an upgrade has to make sense for you and your aircraft mission, you also need to be aware of its impact on the future value of your aircraft.
A clean, well-maintained aircraft will tend to be more reliable in its day-to-day use. Along with its records, this will speak volumes to a future buyer that they are indeed getting a quality aircraft.