One of the things I learned was to weigh stuff.
I am not a gram weenie, but the grams do add up to kilograms and 5 kilos can make the difference between a reasonable hike or a miserable and exhausting slog.

If you take all the trinkets and put them in a bag and weigh them all together you are likely in for a rude surprise.
Small items that weigh nothing tend to add up fast and suddenly you find yourself with 10 lbs of small tools, and usually they all copy each other's functions.

Packaging matters too. I have seen people with half their pack full of the packaging their stuff came in.
That stuff all weighs.

Check how much all the holsters and pouches on stuff weigh and be sure you need them. Often you can get better protection by stowing some gear inside other gear.

For example: My GPS unit is usually either in my pocket or in the pack.When in the pack it needs some protection.
I keep it in my spare socks instead of the special pouch (which stays home).

Another thing to weigh is the pack itself.
You need a certain amount of material to make a strong stable pack. After that every extra bit of pocket, zipper or strap is adding extra weight so you want to be sure they help.
I have two expedition packs here that are the same size but one is twice as heavy as the other. The heavy one is designed to stand up to the abuse of rock climbing.

I always suggest people load their pack and walk about one mile out and back, then get the scales out and start looking.
If you are doing it in a city be sure to include some hills and stairs.