A lot of people ask me about travelling, especially in South-East Asia. I shall dispense this advice forthwith.
Stay a bit longer
How much is it costing you to fly there? How much time will it take to get over jet lag? If you're flying to the other side of the world, try and make it worthwhile.
It's worth considering an extra 2 weeks unpaid vacation and subletting your apartment while you're gone or having your friends coordinate airbnb bookings for you.
Now that you're a bit less rushed, you can travel a bit more slowly. Take the night train between two cities on your itinerary. AirAsia is ridiculously cheap, but nothing beat the $14 bus ride from Phnom Penh to to Ho Chi Minh City with a land crossing. We saw so much more of those countries than we would otherwise have, met interesting people and did I mention how cheap it was?
We also traveled on night buses in Vietnam, another crazy adventure.
Every long-term traveller I've met downsized their packs. Having to carry a suitcase means you're a whole lot less mobile. Getting off a bus at 6AM for a day of sightseeing is a whole lot easier with a small pack.
There's almost nothing you absolutely need to bring with you. The only thing I wasn't able to buy everywhere is deodorant. Either pack your own, or do what the locals do and shower twice a day.
You can buy clothes once you get to your destination, and they will be more adapted to the climate. It's also more fun to buy something at a market than visit yet another temple. Camping style clothes that dry fast are worth it; you can hand wash them at night, hang dry and have them ready for the next day.
Electronics? An unlocked phone is a good idea. A small laptop or tablet if you're an addict like me.
Gifts and all that shit you bought? Mail it.
Not credit cards (but have an extra, warn your bank and write down a number you can call collect *from the countries you'll be in*). Getting into countries can be a drag and depends on your nationality. If possible get them ahead of time, otherwise you can't use some cheaper land transport options.
Vaccines and meds
You can get some good vaccines cheaply in Bangkok, and medications are available just about everywhere in SEA without a prescription (there are even pills for erectile function sold on the streets, near prostitutes). It's a good idea to have medications for most common problems (food poisoning, diarrhea, dehydration), or just their names so you can buy them there if needed.
Stay flexible - only book your return ticket
On my last trip I bought a one-way ticket. If you know when you need to be back, buy a return ticket. Regional tickets are usually cheap, even at the last minute.
While in Cambodia the King Father died and for the national mourning period all bars were closed. People even looked at us funny for daring to smile, so we booked the next bus ticket to Ho Chi Minh City - then found out we didn't have the right visa, rescheduled the bus ticket and went to the embassy in person for a rush visa that cost several times more than the bus ticket. We crossed the border less than 24 hours after our decision to leave.
Floods, revolutions, strikes, outbreaks and typhoons can all happen. Most of the time it's not dangerous though family and friends might worry. I was in Seoul while media were announcing that nuclear war was imminent, and everyone there knew it was fine.
Sometimes it can be something as simple as rain on the days you had planned to be on the beach in Nha Trang, so you leave early to spend more time in Hoi An where you'll get some cheap shirts tailored.
What to see
There are plenty of websites covering each destination. My only advice is to go to a market, buy all the weird fruits and eat in places outside the banana pancake trail.