Unless you're planning on camping, leave the sleeping bag at home and just take a sleeping sheet.
Unless you're planning on climbing a few mountains, leave the trekking boots at home.
Be selective in how much music you bring as you can always buy more.
Quality slide and print film along with cards for digital cameras are available throughout the country.
Traditional guidebooks both new, second-hand and pirated are easily purchased throughout Southeast Asia. Prices may be considerably lower than your home country so consider buying books as you go rather than getting them all before you leave.
Be wary of lugging too much electronic equipment. Do you really need six lenses, two camera bodies, a laptop, a potable music player and a mobile phone?
Planning well is an integral part of getting the most out of your trip. Be it picking the right backpack, the right vaccinations or the right country, the simple decisions are often the most important.