My backpacking guru, Larry Williams, broke it down for me many many years ago. He told me to think of my backpack as a house. I should make a list of the rooms in the house, then go from there...
In the kitchen, I would need pots, camp stove, fuel, eating utensil, etc...
In the refrigerator, I would need the food for the trip.
In the bathroom, I would need my first aid kit , toilet paper, toothbrush/toothpaste, and other stuff.
In the closet, I would need my clothes ( remember rain gear ).
In the bedroom, I would need my sleeping bag, pad (Thermarest is best), bivy bag.
In the living room, I would need my maps, compass, entertainment ( mini starwheel , whiskey ), etc.
Make a list! This house analogy is the best that I've ever heard on "How to Pack". It can be applied to any situation, whether it is a bugout bag, diaper bag, luggage, or backpack.
TIP: Keep your list as a future reference, and go over it once the trip is done to see what can be added or taken off.
TIP: Buy quality light weight gear. Ounces make pounds, and pounds make a happier hike.
TIP: Knowledge is lightweight and you can never carry too much of it.