First, focus on scale. Mix items of different sizes and shapes, and include something tall (usually a lamp), medium (like a picture frame or small painting) and small (a vase or small decorative bowl are good choices).
Then, consider color. You can try mixing objects that are complimenting colors, like blue and brown.
Or you can follow Phoebe Howard's example and stick to a monochromatic color scheme.
Odd numbers are more pleasing to the eye, so limit yourself to three, five or seven things.
Better Homes and Gardens
And last (but certainly not least in my book), mix the old and the new. There is nothing fresher than antiques layered with something modern!