I once moved from KY to NE in one trip in a compact car I bought a week before the move. I was ruthless and did not take anything that could easily be replaced for a few dollars at a thrift store or dollar store at the other end (mostly kitchen wares, wastebaskets, etc.). Renting a truck or trailer, or making two trips, would probably have cost over $100, and that buys a lot of household goods. I took only sentimental items, clothing(very few!) and basic equipment.
My other trick was to not use any boxes for packing. I used only softsided suitcases and bags (duffel bags mostly, but some garbage bags). This allowed me to fill every inch. Boxes (and the resulting empty space in them and between them) take up a lot of room! Books were not boxed, but simply stacked on the floorboards of the car (front and back). Clothing and linens were used for padding anything hard or breakable. Anything with an empty space (like the suitcases, shoes, a pan or jewelry box) was filled with smaller items or nested with other items to make everything compact. For example, my portable sewing machine had a cover to protect the machine, and inside there was a lot of empty space that I filled with my other sewing supplies wrapped in kitchen towels. This helped protect the supplies and the machine, and utilized the space.
Uncluttering is key; but so is a smart packing strategy.
