Make sure you have clean hands.
Use both hands to pick up the brim, either at each side or front and back.
Never pick up the the hat at the crown! It can cause permanent dents.
Store your hat in a cool, dry place.
Place hats in hat boxes (to keep dust off) if you're storing for long periods of time.
Use a hat stand for temporary storage to help keep its shape.
Hats with flat crowns can be stored on the tip (upside down).
Hats that have an upturned or flat brim can be stored flat, right side up.
Over time, crowns that are turned down, asymmetrical, etc., will buckle if not stored properly, so keep the weight off the brim. Stuff the inside of the crown with clean tissue or paper to make an impromptu stand to keep the hat from resting on the brim.
With a measuring tape, start above your eyebrow, measure around to just above your ear, to the largest part of the back of your head and around again, past your other ear, to the front of your forehead. (Tip: to ensure an accurate measurement, try to keep the tape parallel to the floor all the way around your head, as if it were a rigid band.)