Look at the length of the trip. Very short trips (3 days) are different to longer term ones; after a certain length the clothes get washed and re-worn.
None-the-less:
3 Pairs of shoes seems to be the optimum limit. It depends on the location, but a pair of comfortable sneakers or something good fro walking / hiking are almost always essential. If you are going to the beach, a pair of throngs or sandals is recommended. Pack one pair of shoes suitable for formal dining, even if that isn’t planned. And take the specialist shoes, hiking, sailing boots, sporting shoes, only if these are needed.
Remember, you will wear one pair when travelling, so you only need to carry one or two others. It’s best to keep shoes in a bag to separate them from other clothing items. In a pinch, you can buy cheap flip-flops or the equivalent at the destination. Always take shoes that are at least a little worn in.
Go with comfortable clothes in colours that are neutral enough to wear most places. If you have two sets of clothes, wear one whilst the others get washed; this can last you a whole trip.
Socks. If you are in a plane, a pair of compression socks can make all the difference. You’ll only use them twice on the trip, but that’s worth it. Otherwise it can be a good idea to buy socks for the journey that you dispose of afterwards. Dollar shops have packets of socks that work out less than a dollar per pair, the thick type that dry quickly. A dozen identical socks prevent issues from losing half of a matching pair, and if they’re cheap they don’t have to last past the end of the trip.
If it’s a short trip, go with an appropriately small bag. Backpacks with multiple compartments are great; failing that, use a duffle bag with zip locks plastic bags to separate items. A plastic bag for socks, one for underwear, one for tops, one for phone items, one with raingear … etc. Easy to sort though; easy if your bag is searched.
Use clothes that don’t need ironing. Take something suitable for formal dining, even if that is unlikely, and prevent wrinkles by rolling it instead of folding it. Some good quality jackets will keep their press even under severe conditions.
Try packing heavy items first. With a backpack put heavy/bulky item at the bottom; this can make the backpack sit better on your shoulders. Check essentials
If going to a foreign country, consider renting a mobile there. Foreign bills can be expensive. Remember, you might need your own mobile when you return, for arranging pickup from the airport. Give your contacts back home your phone number and other details by email as soon as you can.
Take sunglasses, a hat, a jacket, … etc. It is easy enough to wear these rather than pack them
Have a large plastic bag, either for laundry or keeping luggage dry in a downpour
Wear something comfortable for the journey, but remember that weather conditions can change abruptly.
Mentally picture yourself at the destination, and see if you picture yourself with something you don’t have.
Buy electrical adapters for foreign countries before you go. eBay is good for this. Remember, they usually change the plug, but not the voltage. Find out what is suitable for you.
Earplugs, face masks, noise-cancelling headphones … etc. are useful to some individuals.
Consider a portable clothes line, a small torch, and allergy-free pillow case.
If you can go disposable with any item, seriously consider it. A cheap toothbrush won’t hurt in the short term; forget electric ones unless it’s a long trip. Buy a cheap watch, a raincoat, a bath-towel, a second-hand book, or two; use sample-size toothpaste, sunblock and other items. Buy some cheap t-shirts. Take old clothes that you won’t bring home. Remember, you probably have less space on the return journey with souvenirs and gifts… .etc. You may need the extra room that disposable items give you.
Passports, tickets and similar items need to be safe, but accessible. You will need them when getting out of a plane, so keep them with you and not with check-in luggage.