I have traveled with carry on luggage for trips as long as 14 days. I just came back from Italy where DH and I traveled with carry on only for 8 days.
When I travel carry on only I roughly follow the 5-4-3-2-1 method. (5 tops, 4 bottoms, 3 pairs of shoes, 2 dresses or jackets, and 1 accessory set).
I also plan to repeat outfits. For a 14 day trip I repeat everything except for underwear and socks at least twice. I use a travel size spray bottle of Febreeze to avoid doing laundry.
When I carry on only, I use two bags, an actual carry on size bag and a personal bag (the personal bag is a tote bag and inside of that I put my purse for the trip). My carry on bag is an actual BAG (think duffel bag), not a suitcase. When I need to maximize space, a bag allows for a lot more room, than a roller suitcase will (because the suitcase handle takes up a lot of room in the suitcase). Of course, this does mean that you'll be carrying the bag on your shoulder in the airport so that is something to consider.
A key for me is to limit my shoes to only 3 and I wear the largest on the plane. More than this takes up too much room, less than this and my feet can't get the break they need (I don't like to wear the same shoe everyday). For the Italy trip I knew it would be a lot of walking so I took 2 pairs of fashion sneakers and 1 pair of flat shoes for evenings.
I fold clothes into squares (think military folding or Marie Condo folding methods) AND use packing cubes. Cubes help keep everything organized and they remove the air when you squeeze them to zip them up. This really helps to fit more items into a limited space. It also makes packing into a bag easier because everything is very compact and you just stack them into the bag. I got my packing cubes from TJMaxx.
I bring a limited color palette. For Italy I stuck with 3 colors (light and dark blue and white). Everything can be worn with everything for maximum variety.
I always think layers and wear (or carry) the largest layer on the plane. That's usually a hoodie, jacket, or blazer for me.
Don't forget to use the personal bag (for me that's a tote bag). I put electronics and toiletries in the personal bag. The rest of the bag is left empty so that I have somewhere to put souvenirs.
Use the bag extender zippers. My duffel bags have extender zippers. I pack with the zippers closed on the flight to the destination and open the zippers on the return trip so again I have somewhere to put souvenirs.
Good luck!
