This question originally appeared on Quora.
Answer by Thomas Snerdley,
Here is 44 years of travel advice from a military brat and a military veteran who's been around the world a time or three.
The Two Bag System. Every pro traveler I've ever met travels light using some variation of this system.
Travel bag : High quality 25 to 40 liter travel pack with (preferably fold-away) hip belt. Clothes, electronics, toiletries, gifts. Liquids/gels in a quart Ziploc by itself for easy inspection removal. My clothes are rolled up into an Eagle Creek 2-sided travel cube [1a] with a plastic divider in the middle to separate the clean from the unwashed (isolating the smell away from the clean clothes and the rest of the bag) without having to pack two separate clothes bags ... genius! Always leave the travel bag at least 1/3 empty for souvenirs, and to leave room to carry:
Purse/man-bag or hiking waist-pack . Everything you will need to survive up to 18 hours in your airplane, train, boat, bus or tuktuk seat (e.g., iPad, smartphone, snacks, eyeshade, earplugs, wet wipes, water bottle, etc). You never have to touch your main travel bag after putting it in the overhead bin.
Going through Security: Put the small bag inside the big bag. Put everything except your picture ID and boarding pass into one of the big bag's zippered pockets and secure it with a heavy duty twist-tie (I use Nite Ize gear ties ). Since your Ziploc bag with liquids/gels is in its own pocket (right?), you won't accidentally lose anything taking it out for inspection. After clearing security, it'll take 30 seconds to put the Ziploc bag back in, your shoes back on, and be on your way. Walk until you find a nice quiet area of the airport away from potential pickpockets to put everything back in your pants pockets. Walk by the folks paying $4.50 for a small bottle of water and, without being smug, fill your empty water bottle from a drinking fountain. (Not sure if the water in that country is safe to drink? Just check http://www.canidrinkthewater.org/)
How do I know what to bring? As you pack your bags, make a text list on your smartphone of everything you pack. On your trip, once in a while when you're bored waiting for a bus or plane go through the list and put a + next to everything you actually used. Next time you travel, don't pack anything without a + next to it. Even after all these years, my travel pack gets a little lighter every time I travel. That's important, because if it's too early to check in to a hotel, I can actually walk around town all day with my light pack (with at least 75% of the weight resting on the hip belt, not the shoulder straps!) unlike "overpackers." 
How do I wash my clothes, for free? In the shower with hotel soap and/or shampoo, while you're still wearing them. That's why world travelers swear by quick-dry Marmot, Icebreaker, and Ex-Officio clothing. Start with t-shirt, socks, shorts, undies, then yourself. After your shower, first wring them out as dry as you can without overly stressing the fabric. Then, lay a hotel towel out on the bed, put one article of clothing on it at a time, roll it up as tightly as possible, then sit on it for a few seconds.  This gets them much drier than hand-wringing alone. Your clothes will now easily air dry overnight draped over the furniture even if there's no fan or A/C.
How do I figure out where to stay and what to do, for free? Before you travel: WIKITRAVEL or Quora. After you travel: ask people. My personal system is to upload Wikitravel pages to my Evernote account, which I have set to automatically download to my iPhone so I can use them even with no Internet connection. No muss, no fuss ...
How do I make friends in a strange city or country if I don't know anyone and I'm an introvert (that's me!)? Sign up for a several-hour activity of some kind through your hotel, hostel, or whoever's offering one. Elephant riding, bamboo raft ride, parasailing, ziplining, hot springs soak, cooking class ... doesn't matter, whatever floats your boat. You'll inevitably meet people with whom you have something in common, strike up a conversation, and gather serious intel on this location (since they've probably been there a day or two) and lots of other places they just visited that they can't wait to tell you about and show you pictures of themselves standing in front of.
Always remember, the only hard part about travel is buying that first airplane ticket! Everything after that is easy.
My current travel setup:
 Boreas Bolinas 30 liter waterproof travel bag.
[1a] Eagle Creek 2-sided cube.
 Chrome Industries Sling.
 Nite Ize gear ties
 Rolling clothes in a hotel bath towel