|15 days, 3 climates, 42L bag + carry-on!|
Anything denim is bulky and difficult to pack. If you're going somewhere cold, maybe wear 1 pair of jeans and pack one more. Denim is super easy to spot-clean, so if you spill soup at dinner or drip ice cream in the afternoon.... it'll be super easy to treat and dry your denim item overnight in the hotel/hostel.
Too many shoes.
This is one thing that requires a little bit of extra planning. If you're planning to hike and beach and go out or a nice dinner.... limiting shoes may seem difficult. I would say never bring more than 3 pairs (1 on your feet and 2 in the suitcase). Make sure your footwear can multitask. There is no shame in wearing your hikers to the beach-- you're just going to take them off once you hit the sand! And odds are you're not going to end up somewhere so fancy that would judge you for your flippyfloppies.
Do your research before you go. Most hostels and hotels include bath towels, and some hotels offer even offer complementary beach towels for guest use. When I go for long trips, I pack a small hand towel just in case. Some places in Asia usually opt for tiny towels, and it is surprisingly easy to manage without the megatowels we use in the Western world. If you're going to be beaching, invest in a small towel and a sarong- BOOM! Something to lay on and something to dry yourself with!
Earrings, bracelets, expensive camera lenses (unless you're a travel photographer), computers, etc... You won't need it. No.... shhh... shhhhhhhhh no. You won't. You think you'll "get some work done" or need it to book accommodations and transportation-- but you won't. I was able to find an internet cafe on this tiny, barely inhabited island off Thailand, so I'm sure you'll be fine. I limit myself to: phone, camera, and maybe my tablet. All jewelry is costume and all (ok, most) clothing disposable.
Too many toiletries.
Try to keep them under 3fl oz and limit them to only what you know you can't find at your destination. Sometimes plans change and you decide to take a puddle-jumper to a small island and cant check your bag-- so keeping all toiletries under 3fl oz will ensure you can remain flexible without needing to throw stuff away. You'll certainly be able to purchase (if its not included in your accommodations) suitable shampoo, conditioner, and sunscreen when you get there. Things like face wash, deodorant, and contact lens solution are the 3 thins I always bring with me... everything else is negotiable.
Too many clothes.
Especially when you're travelling to warmer places, the impulse to over-pack can be strong. The rationalization of "but these clothes are smaller, so I can pack more!" is super overwhelming. Think of the activities you'll be doing. If I'm unsure, I usually have 1 outfit that can be worn on super-active days, 1 nice-ish dress/outfit, and the rest is just day-to-day clothes. It can be particularly difficult when you're going somewhere cool or cold- you just have to edit your packing list. Remember: as long as you have enough clean underwear, you're all set :)
More than 1 sweater or jacket.
I usually have a light jacket and a sweater. I purchased a super lightweight jacket that is really easy to pack, so I don't feel bad about having the extra sweater. Even warm places get cool at night and I don't love super-cold AC in hotel rooms.
Other things to note:
You might need an umbrella if you're going somewhere wet. Don't pack one "just because"... because you may then forget your favorite 'brelly in Bali. Whoopsie! Check the weather and make a judgement call.
Medications. If you are prone to headaches or have allergies, then you might want to stash a few doses of whatever brand you prefer. Yes, you can probably buy ibuprofen or antacid at destination, but it'll likely be in a different language, and meds are not something I would gamble with.
I love my guidebooks, but they can be a drag to pack. Personally, I would rather swap out 2 shirts to be able to bring my guide, but not everyone feels that way. Odds are good that you'll be able to ask someone or look on your smartphone to get the info or maps. Or, if you're like me (and yes... I do this even if I'm bringing the book) I make a list of all the places I want to visit, so I don't have to thumb through the book between each stop.