When you buy things that are expensive, like a sofa or something that really feels like an investment, you need to keep it as plain as possible, as simple as possible. Make sure that it's a clean design that will work with whatever style you want it to.
My favorite thing about decorating is mixing different periods and styles. If you have something that's old, and you really do want to mix those styles, then you have to add something that's obviously modern with it. You can't put a kind of a mediocre thing in the middle.
