Thanks!
I actually learned it from my mom. I think she would repack the car before every vacation because my dad was insistent that it wouldn't all fit. It always did.
One of the ways I can get so much stuff in there is the type of bags I use. I try to use more small bags for things that are soft, so I can cram them in. Last year, I bought a huge duffel bag, but it's the perfect size to fit in the boot with the seats up, if necessary. Also, I rarely get it right on the first try.
I'm happy to help anyone pack!
I know everyone makes fun of me for how much stuff I take, but I can pack suitcases really well, too. I only took an overnight-bag sized carry on for a 3 night trip to California a couple of years ago. I knew no one would believe, so there's a picture on Facebook somewhere.