For me, in the hot early days of our season, I might just wear a Core top and the Ascent pants.
As it begins to get cooler, I just start to add layers. Maybe a Traverse Zip-T (or the '07 Mountain Shirt) goes on next, topped by softshell outer layer like Ascent Jacket (warm) or 90% Jacket (cooler). As temps continue to get colder, you add the Vest, and then maybe the Celsius Jacket. And then when the temps really drop, throw the Kelvin Jacket on as the next-to-last layer (with Celsius on top).
For the bottoms, when it gets a hair cooler, you substitute the Ascent pant for the 90% pant. Then add the Core bottom next-to-skin for a little extra help, and then substitute the Traverse bottom for the Core when it gets even colder. The Celsius Bib goes on over the 90% or Ascent (and one of the core layers) when it gets really cold, and then when the bottom drops out, the Kelvin goes on under the Bibs.
For hat I start with the ball cap, or beanie when I don't need shade on my face. The beanie goes under the ball cap great as well so you can have insulation without bulk as well as shade. And then when colder I substitute the Celsius hat for the beanie, and when really cold put the Traverse Balaclava under the beanie or the Celsius or both.
I do not have any of the more waterproof/insulative/expensive layers (like Stratus) except for the older model Downpour Jacket/Pants for raingear.
It sounds a lot more complicated than it is. One of the things I've liked best is that if you decide to go with the Sitka layering system, I really think it actually makes your decision-making easier. It's easier to decide what to take with you on a trip. Take the whole system and you're good for whatever weather dishes out. It's easier to decide what camo to wear--whether you chose the old Mountain Mimicry (a GREAT pattern) or one of the two new Optifades, you are going to blend in almost anywhere. The stuff packs down light and tight, and will last. And this time of year is a good time to check out dealers who carry Sitka, who often will have it marked down substantially to make room for the '11 gear.