Finally, leave some weight and space allowance for a medium-sized, very light-weight collapsible extra bag that you can carry folded-up in your primary bag.
Yes, you may have to pay the airline more to check this but it can be your life-saver for all the extra weight you will pick up from shopping or travel literature you want to take home, etc.
You won't want to put breakable souvenirs in this bag but instead you might use it for your dirty clothes, carefully packing the more breakable items in your main bag surrounded by your remaining clothing for protection.
Reserve the most fragile and valuable items for your carry-on bag. TravelSmith carries something like this called an 'Excess Baggage Tote' in TRAVEL SUPPLIES - FLIGHT FRIENDLY GEAR (on their site). Go to Stores for Travelers