Most of this is rural/camping based, but the principles extend to less basic trips and accommodation:
Rule Number One: If you can't carry it on your back - and be able to sit down and take off/put back on your pack without being pulled over like a stranded beetle - you've packed too much.
Douglas Adams was right - you should always know where your towel is. Wrapped in a waterproof bag and secured on top of your stuff, underneath the top flap.
Separate pouches inside the pack are great - if you have different colours, it's really easy to pull out the one you need, rather than having black which you can't find easily in the depths of a pack, especially when it's dark. Include one for dirty clothes, so you don't have to smell your own socks all week sort through what you've got that's clean.
Split out an emergency 'It's cold/pissing down/the tent's blown away' pouch which contains a woolly hat or buff, pair of socks, some big knickers, preferably the shorts type, lightweight longsleeved base layer, sports leggings and cheapie pac-a-mac, along with a foil blanket, some 3x5 inch dressings (perfect for grazed knees, covering heels properly, hot spots on your sole or side of the foot) and a couple of high energy snack bars. Equally useful if you find yourself stranded somewhere unexpected due to transport issues and you need a pyjama alternative as it is for tent related troubles.
If you've looped the laces together and attached a pair of boots to the mid point of your pack with a carabina, shove a pair of socks in each one in plastic bags, as this means you a) have another two pairs of socks and b) if it rains, the inside of the boots stay dry.
Summer weight buffs are cheap as anything, pack down to absolutely nothing even if you don't have them on your wrists and have multiple uses,, including head coverings for hot or cold, bags, neck coverings, arm coverings, sweat bands, bandages, slings, flannels, bag handles and hand warmers.
Much can be done in an emergency with gaffer tape and a couple of bin bags.
Pack sun block, shower gel and more sunblock (you really can't have too much) on the side pockets so they're always easy to access and if they get crushed, it's only the outside of the bag that is potentially covered with factor 50, not all your clothes.
There is often a small internal pocket on top of the usual ones - put a spare house key in there. The very top pocket on the pack needs paracetamol, tick grabber if you're going somewhere grassy (including parks), head torch, couple of dressings, saline wipes, a buff and is a good place for tinted lip balm, toothpaste, toothbrush, handful of hair bands and a mini bottle of 2-in-1 shampoo and conditioner.
Just in case it all gets too shit and you decamp to a hotel, have a lightweight jersey fabric dress at the bottom so you can have a shower and change into something comfortable.