We've been traveling more than usual in this last year and I am happy to report that I have stumbled upon the best secret for better suitcase packing ever. You will need a box of plastic bags, the ones that close with twist ties. Just a plain plastic bag. Sometimes they are called bread bags.
Now, instead of folding your clothes, roll them.
(These are my daughter's clothes. And I was packing for a trip to Disney, hence the Jessie the Cowgirl outfit.) (Not that she doesn't dress as Jessie the Cowgirl when we are headed somewhere like the grocery store.)
Then slide the clothes into the plastic bag. I can fit three or four of my own shirts in one bag. For kid's clothes, I pack an entire day's outfit in the one bag and use a separate bag for each day.
(Photos from: me)
The plastic bag will keep your clothes tightly rolled and you will be able to fit much more into the same suitcase. Rolling the clothes, as long as you are careful when you do it, also keeps them wrinkle free. By using this technique, I was able to pack everything I needed and everything my toddler needed for a six day trip into one carry-on suitcase. And I even had room left over for my giant baby video monitor which apparently I can not leave home without.