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If your cakes are tougher and drier than you'd like, you're probably beating the batter after you've added the dry ingredients.
Absolutely beat the heck out of sugar and butter (eggs, too, unless using beaten whites); but if you beat the flour--particularly all-purpose flour--you'll develop the gluten, leading to a tough, dry result.
Cake flour doesn't contain as much gluten to begin with, but unless a recipe specifically calls for a lot of beating once you've incorporated the dry ingredients (usually only thin batters require it), try folding them into the creamed liquid ingredients. Your cakes will have a finer crumb and a moister texture.