Think Parisian apartment, not suburban McMansion. And remember that a smaller house means there is less to clean.
Keep the scale of the furniture appropriate to the scale of your house. As tempting as that huge sectional is, don't get it. A smaller sofa and some chairs will fit better. Furniture that is "lighter" will help, too. Skip the skirts on the sofa and chairs; being able to see the legs will add more visual light and space.
Quality over quantity. This goes for clothing, shoes, toys, kitchen tools and gadgets, etc.
Elfa. You can get a lot more usable storage out of a standard closet if you install Elfa. If you have a garage, get industrial-type shelving units and store as much as you can out there. Depending on the ceiling height, you might be able to do shelving across, too, or hang bicycles, etc.
Can you add more shelves to your kitchen cabinets? If they aren't fixed shelves, but adjustable, you can add more. Measure them and go to Home Depot or Lowes for new ones. Made a big difference in our "pantry" and other cabinets, and not terribly expensive.
Don't shop at Costco, unless you are splitting the goods with another family (or several). Who has the storage space for those quantities?
Underbed storage boxes. LOVE these!
Simplify other things. I've started keeping just one or two rolls of gender-neutral wrapping papers. Easier to swap out the ribbons for different looks for different occasions. Glossy white wrapping paper is really pretty, IMHO.