Timing is Everything
Create a timeline when possible. Start doing a little now, don't keep delaying the process.
Choose Quality over Quantity
Enjoy the finer things in life! (Example: Instead of keeping 10 sets of sheets, keep one (or two) with the highest thread count.)
Work In Stages
To avoid overwhelm, approach the downsize by layers or by room.
Live It Up
By removing excess (possessions) in your life, you create a calmness and have more time and energy for things you enjoy.
Consult a Move Expert
Just like you seek advice from a financial planner, doctor, lawyer, etc., seek help from a move manager. They can coordinate all the details of a move on your behalf and greatly shorten the duration (and stress) of a move.