Laundry detergent usually says "Fill cap to line 1," but it's impossible to see line 1, so we just fill up the cap, right? This uses 4x as much soap as you need! Take a permanent marker and mark line 1 inside the cap (you'll need a bright light to find it). Only fill to that line -- even for extra-large loads. Your clothes will get just as clean, and you'll save tons of money!
For powdered detergents, mark the measuring scoop the same way -- or use an old 1/4 cup measure as a scoop.
For front-loading washers, use even less soap (about 2 T)!