Before I set off to backpack around SE Asia, I did a lot of research on what to take, and what not to take. Some of the tips from my research turned out to be brilliant, and some a waste of time. Here's the run-down:
7 things to pack
1) A small role of duct-tape
I bought one of those mini rolls (with a very small internal cardboard tube) designed for travellers, and used it multiple times. I've repaired lots of holes in mosquito screens, as well as the odd rip in a mosquito net. Worth the space in the backpack.
2) A tube of anti-itch cream (I used Anthisan)
No matter how much I tried to avoid mosquito bites (long trousers and socks after dusk, covering myself in deet) my ankles still got bitten constantly. If I couldn't avoid the bites, at least I could avoid lying awake all night itching.
3) An unlocked phone, so you can buy a local sim card (I bought one in Thailand and another in Vietnam)
I'd originally planned to stay in touch with family back home via skype, but found that the wifi connections weren't always fast enough to be able to use skype effectively. So that phone ended up getting quite a bit of use. And, of course, it also functions as diary, alarm clock, and music player.
4) A sarong
It got used as a skirt, shawl, beach cover up, towel, and sheet liner. Definitely worth taking.
5) A thin sleeping bag liner
I didn't bother taking one of these, and wished I had. Sometimes, I just wasn't sure about the sheets, as some places don't seem to iron them. They may be clean, but it's a lot harder to tell if they're wrinkled. I ended up buying a flat sheet half-way through the trip, that I could use whenever I had doubts. A silk sleeping bag liner would have been a lot smaller and lighter to carry around.
Not only prevents those annoying headaches you get from too much sun, but is good for hiding your sweaty hair (I don't think I've ever sweated as much as I did those first couple of weeks, when I was still trying to acclimatise to the humidity).
You will use them. Trust me.
5 things not to bother packing
1) Torch / flashlight
I took a small, wind-up torch, but only ended up using it a couple of times. If you're planning on camping or spending a lot of time on the sort of small island where the power gets turned off every night at 22:00, a torch would be a good idea. But apart from that, I'll probably skip the torch next time and just use the light of my phone.
Next time I won't bother. I'm not a beach-bum, but I did visit several beach locations on the trip (Prachup Khiri Khan, Langkawi, Cherating). I still only wore my swimsuit a couple of times. Half the beaches I visited were more geared to locals then farang, so I swam in shorts and a t-shirt as often as not. And even on the more touristy beaches, I wouldn't have felt out of place in shorts and a t-shirt, so next time I'm not going to bother packing the swimsuit.
They can be bought easily and cheaply anywhere, although you may have to search a little to find sensitive skin formulations (even the baby soap in Thailand seems to contain perfume).
4) Mosquito nets
Unless you're planning on camping or staying in really, really basic accommodation, you won't actually need to bring your own mosquito net, and mosquito coils can be bought easily when you're there.
5) Travel sink plug
I took one of these so I could do wash clothes in my room (when I just want to wash one or two things, so don't want to send out a load of laundry), then found it rarely fitted properly – a complete waste of space.
One more thing to remember
Clothes wear out really fast on the road, far faster than you can imagine.
I went with one pair of thin linen trousers (which were a couple of years old but still looked in decent condition), one ankle-length skirt, one pair of shorts, one long-sleeved shirt and three t-shirts. One month into the trip, the trousers had holes and threadbare patches, so I binned them (and being that I'm almost six foot tall and wear a size 16, finding replacement clothes in Asia was not an easy task). By the end of the trip, the skirt and one of the shirts were bin-worthy as well.
Pack lightly, but pack quality, and don't expect the clothes to outlast the trip.