This is the hardest thing to decide before heading off on a big trip and it is one that caused us the most anguish. We spent a long time planning what to pack and I have to say that the preparations were worth it. We got it spot on. If your trip involves jumping in and out of countries then its even more complicated as you need a variety of clothes, shoes etc. However the best decision we made and I would absolutely recommend it as a top tip is to BUY A SMALL BACKPACK !!!! By small I mean 40L or else. Mine was 38L and it was more than enough. If you buy a bigger pack, you will fill it and have to carry it. If you don't have the space, you will make do. You can get almost everything as your travelling so I would only buy good essential items and then pick up others and give away unwanted clothing as you go along.
Recommended Packing List for all sorts of climates
1) BACKPACK(maximum size 40L): Spend time picking this out and make sure its fits correctly. Buy a brand that will last. I had a 38L Gregory pack and it was fantastic. Ger had a Deuter and it was brill too. You might also want a small daypack or handbag that you can put securely around you.
2) TRAIL RUNNERS OR HIKING BOOTS. We had trail runners (salomon) because we wanted to do running too but if your hiking/walking then a good pair of hiking boots would do.
3) WALKING SANDALS for walking around everyday. you can pick up flip flops later as you go around too.
4) 1 X TREKKING TROUSERS, you might want ones that covert to shorts or that roll up so you can use them in hot weather too. The long ones are great for evenings when insects are out.
5) 1 X Shorts/Trekking skirt/3 quarter length trousers
6) 1 x Swimming Suit/Bikini. I'd also bring goggles if you plan on doing a lot or a snorkeling set if you plan on doing a lot of it at the start of the trip. The rental ones are not great at times and you could sell it on or give it to someone when your finished with it.
7) 1 x Fleece or Jumper. Fleece is good.
8) 3 X T-shirts/stringy tops. If your going to Muslim countries, you wont wear stringy tops on their own so t-shirts might be better
9) 1 x Sarong. Get one that you can use as a wrap/skirt or just to sit on. I had one and found it invaluable....especially when I wanted to cover up from the sun or avoid insect bites.
10) Underwear: get some quick dry underwear and some good quality trekking socks. Buy good quality as this can be harder to find as you go around.
11) 1 x Skirt/dress. At times you might want to be a bit dressier so I would bring a maxi dress or something that you can wear both day and night.
11) Buff for neck and head. These are very handy to have.
12) Jacket: A windproof/waterproof is good. The soft-shell windproof brilliant for trekking but you will need a lighter rain jacket then too. Or you can splash out on a Gore-Tex one that does everything. We bought our heavier jackets as we went along as we didn’t need one for the first few months. I only had a light raincoat. Its depends where you start.
13) Rash Vest: this is great to have for snorkeling, swimming and keeping the sun off. Its hard to get so I'd get one before you leave.
14) Toiletries..keep it to a minimum, they weigh so much and you can top them up as you go along
15) Electronics: we had a camera, waterproof camera (can really recommend the Kodak play sport for this, small laptop and kindle (it will be your best friend), universal world plug.
16) Essential Kit Items: Head torch, universal sink plug, basic first aid kit (you can top this up as you go along), dry bag (maybe a couple of these). Store electronics plus etc in dry bag. safety belt for money etc.
17) Essential Paperwork: Passport, drivers license, ticket print out etc.