The fine art of packing is something I don’t think I will ever master, but here is my approach:
Pack for a week, even if the trip is longer. That helps to keep the amount down, but you will have to do laundry along the way. I wash items out as I go, usually a few things each time I take a shower.
Before I zip up the bag, I do a mental toe to head check to make sure I included at least some shoes, socks, pants, underwear, shirts, bras. This has worked well in that I’ve never left a whole category of items although I have on many a trips forgotten a hat. Even better is when I actually compare what I have in the bag to a real packing list. The calm of not waiting until the last minute is worth it (even if I have to repeatedly learn that lesson).
Take less than you think you’ll need, really. The fun can be making do with what you have even if it leads to outfit combinations you wouldn’t normally wear.
Packing cubes are awesome. I usually put shirts in my biggest one; bras in one and socks and underwear in a third.
Layers really do help, and even if going someplace warm, make sure to take at least one warm long-sleeved item. You never know if there will be too much AC or the temperature drops at night or it is unseasonably cold. Often I take it on the plane with me and am glad I have it even if I don’t end up wearing it other times on the trip.
I always take a swimsuit.
I like my own supply of over the counter medicines to include ibuprofen and Pepto.
Zip lock bags are endlessly useful so I add a few extras. I gather all my electronics cords into one zip lock.
Make sure the item is in the case. I once packed my camera bag but left the camera on the coffee table in the living room.
Triple-check that you have money, passport, contacts and eyeglasses (or whatever your absolutely must have essentials are.)