So, what content is in this wonderful bag of mine? Clothing, a laptop, a notebook, and a few toiletries. For my trip, I packed:
3 shirts (2 t-shirts, 1 button-up)
2 pairs of shorts
1 pair of long pants
1 pair of pajama pants
5 pairs of underwear
2 pairs of socks
1 pair of shoes
This is more than enough clothing to travel with. It is very inexpensive to do laundry in Southeast Asia ($2-4 per load). In my opinion, washing your clothes on a frequent basis is FAR less annoying than carrying extraneous weight.
Some days I wish my laptop was lighter. It doesn't seem that heavy until you factor in a charger and surge protector, then add it to your increasingly widening backpack. I love my durable Thinkpad. However, a small netbook or even Ipad will suit the average traveler perfectly.
Toiletries should be kept to a minimum. It is actually cheaper to buy most toiletries in Southeast Asia, so don't bring soap, shampoo, deodorant and tubes of toothpaste (well, maybe a small one for the plane). Besides the fact that you will save money, it will also force you to choose what your essential items are. Do you have room for hair-styling gel? Depends on how much of a strain your back is feeling.
Other "essentials" to leave at home: a towel (most guesthouses/hotels provide one free of charge), blanket (same), and pillow (unless it's one of those comfy ones for your neck). I learned my lesson from visiting Israel back in January. I did my back no favors, and vowed to pack just the bare minimum for Southeast Asia. This is a very hot region of the world, so you won't be needing coats, jackets, sweaters, etc.
Finally, if you'd like to carry a book with you, keep it to ONE book at a time! You can always trade it in to a used bookstore and buy a new one. Right now I have three books in my bag (as soon as I finish this monstrosity of a J.K. Rowling novel, off to the bookstore it goes), and they add a lot of unnecessary weight.