Here's my process...
Start by laying everything out. Of course it's different for South East Asia than it is for Europe in winter.
For this 12 day trip I have:
4 pairs pants (Jeans, Black wool pants (dressy for dinner), grey cords and black Dockers)
4 cashmere sweaters (warm but lightweight-Blue, Burgundy, Green and Black)
4 long sleeve t-shirts
2 short sleeve t-shirts
1 dressy shirt
2 pair Black slip-on walking shoes (loafers and trainers)
1 pair Black boots with 2" heel
Bathing suit and flip-flops for spas
assorted undies and socks
silk long underwear
Black wool coat
Gloves and Scarf
I put it all in large zip lock bags and squeeze the air out and it comes down to this with room to spare...
What I'm wearing on the plane is to the left. I also take a t-shirt, undies and socks in a small zip lock bag in my smaller carry-on just in case I have to check the roll-aboard and my luggage gets lost or delayed (it's happened!).
All of the other misc stuff goes into the smaller carry-on bag except guidebooks for later in the trip and a small day bag. I make sure that all toiletries, and electronics (see tech toys below) are never checked. Hint: never separate your charger from your camera- it's a recipe for disaster.
When all is said and done, here's what I have for the trip: