Create or load up a flight with aircraft, airport, weather of your choice. Once you're in the aircraft set any aircraft conditions you like...engines and systems off, parking brake set, etc.

Save the flight, but..... In the save flight window, put a check mark in the box next to 'Make this the default flight' before clicking OK to save.  When you start the sim this will be the flight displayed. You can then change the aircraft, airport, weather and time to anything you like and the aircraft will be in the condition you saved.

This only effects flights you create in the Free Flight window.. It will not change the aircraft conditions in flights you 'Load' from your other saved flights. For those you would have to set the conditions and (re)save.

Note: this, like a lot of other information, is covered in the FSX Learning Center. see the 'All About Flights' main topic.