The gear you need depends on your "mission". If space is extremely limited, or you will not be in any single place for long, you'll really need to clamp down on what you carry. On the other hand, if space is of little consequence, then you've got a lot more leeway.
Things to always carry
There are a few things I always try to have with me on every and any trip. Time and time again the space they consume is justified the first time I need it. The concept here is to meet all my basic essentials without carrying stuff I'll never use.
My base bag is a Tom Bihn Aeronaut 45. It's the maximum size carry-on travel duffel bag that can be brought onto an airplane. It's got a heavy duty cordura exterior, and can be carried three ways: shoulder strap, hand carried, or my favorite: like a backpack. In it I carry at least one full change of clothes, a flashlight, a Cabeau neck pillow and eye mask, a First Aid Kit with a full compliment of blister-pack pill-form over-the-counters like ibuprofen, acetaminophen, laxatives, anti-diarrhea, and antiemetic. I'm also kind of a dork and carry around a Cammenga military-type lenstatic compass.
In a smaller travel pouch, I keep all of my charging cables, chargers, and small accessories that would get swallowed up by my bag or otherwise lost.
These things alone get me through 3-5 days of travel. The plus of traveling this light is there is no need to check any bags, so there are zero chances of losing luggage or baggage delays. I walk right off the plane to my transportation.
Little space or moving every ~30 days
I will sing the praises of Pelican Cases all day. They're unbeatable. They're also expensive, but did I mention they're unbeatable?
You have to be careful, though because it will be easy to incur airline overweight baggage charges if you don't plan carefully. Pelican now makes the 1637 model which is marketed as the Pelican Air line. They're about 40% lighter than an equivalent-sized case so you can get a few more pounds of stuff inside without going immediately overweight. The total interior space of the 1637 model is 90 liters, so it triples your total baggage space if you have a 45 liter bag like the Tom Bihn.
In this larger bag goes all of my additional clothing, shoes, boots, flip-flops, books, whatever I think I'll need, usually to stave off boredom.
Trailering or long-term boating
It's easy to overpack especially in these cases, but you do have a little more leeway. On our last couple of trips I've brought nearly all of my tools. On the first trip I did so to get them organized, and I did. On our second, month-long trip with my wife and two little girls, I brought them again in case I had to make repairs. On both trips I had to move all those tool boxes into our truck for (relative) safekeeping since there was definitely no space in the trailer for all of it. This won't be a problem on large vehicles or vessels, but our little 26' travel trailer (and Shiela) would have none of it.
In the end
Carry only what you think you'll need. I know you'll think you'll need everything, but really you can get along just fine with much less than you think.