Layers and high-tech fabrics are definitely the key. I often travel RTW with carry-on baggage (so less than 35 lbs) and temps from 100F to -20F on the same trip (!!). Additionally need formal clothes, but that is outside of this topic. I use Duofold for thermal layering, polar fleece jacket (real polar fleece, to support the factory
http://www.polartec.com/polarfleece/ ), and goretex outer jacket (mine is Eddie Bauer EBTEK which is rated rainproof for 1 hour under a firehose). Cost is surprisingly modest (USD 300 would get you several changes of clothing). I also have goretex shoes (good for stomping through puddles) and goretex socks (rarely needed). The other advantage is most of these clothes are drip dry, making laundry on the road much easier when there isn't time for overnight laundering.
Edit to add that windproof might be the key consideration in a jacket; look for "wind blocker" designation, or similar. Often seemingly identical jackets which are otherwise ideal will differ between their degree of windproofness. I have a down ski jacket which is rated for -50F or something like that, and it is no warmer than my goretex uninsulated coat when combined with polar fleece layering.