Short answer
Start with the garage and attic, measure the new home, decide piece by piece, and schedule removal in phases so decisions never feel rushed.
Start where it's least emotional
Garages and attics tend to hold fewer daily-use items.
Measure the new space
Floor plans make furniture decisions objective.
Phase the removal
One visit before the move and one after is common.
Measure before you decide
Get the floor plan of the new home and measure the rooms. Furniture that won't fit should be removed before moving day so movers aren't paid to transport it.
Pace the process
Start with storage areas weeks ahead, then book one final haul after the move for anything left in the old home.