When I fly, I use the largest possible carry on backpack...and nothing else. It was enough for two weeks in Thailand.
I did have to have my laundry done in the middle of the trip, but in Thailand that's cheap. :-)
Woolite, the traveler's best friend. Did the laundry in the sink, hung the clothes in the hotel room and generally they dried in 24 hours. Took a single carry-on backpack for a two-week trip to Spain and Italy and did just fine.
I recently spent 10 days in Montana for work.
I could have fit my normal stuff easily, but I also went skiing one day and needed some extra gear. I packed it in a Rubbermaid box and shipped it. Far cheaper and easier than checking bags.
Normally I go a week or so on a carry on.