Rick Steves has some good techniques on traveling light--- nothing as radical as UL hiking.
I like the "threes" method for clothing-- one to wear, one clean, one dirty. Works good for socks and briefs anyway. Pants can be stretched more. A couple no-iron polyester button down shirts work for me. Synthetic tees are fine for casual. You can always pick up spares along the way.
Umbrellas are handy.
Warm climates make it all easy. I could do Hawaii with a Flash 18. Sandals mean no dirty socks. Quick dry shorts and shirts, sunscreen and a book.
Some UL gear swaps right over-- rain gear comes to mind. Certainly the philosophy works-- take only what you will use, multiple use gets more points, layering still does the trick, etc.
I like panel loading packs for travel. You can fold clothing better and they are easy to work from in your hotel room. Easier for customs too.
We did three weeks in Europe in the dead of Winter with simple carry-on convertible bags (read COLD and more clothing). You don't need a full-blown suspension, but pack straps are nice when you want to do a quick 6 block walk from train station to hotel and a shoulder bag/suitcase is a pain. I got a killer Eagle Creek convertible for $10 in a thrift store. Not UL, but very good for travel and bombproof.
Definitely *one* bag. It makes it so easy on transportation, climbing in and out of cabs, buses, trains-- no baggage claim or lost bags. If you are doing hotels, a stowable day pack or shoulder bag is great for day trips where you want to haul a camera, map. guide book, water bottle, snack and a jacket/raincoat.
Ditto on stashing money. I keep a "throw-away" wallet with a little local currency and "fake" plastic like membership cards. The big stuff is stashed out of sight.
I wouldn't carry any cash or documents in my backpack-- it's the first thing to get lost or stolen.