Pick fabrics with patterns in different scale; if you choose an large medallion print as your inspiration fabric, choose, for example, a wide stripe, small floral and a textured pattern on your other pieces.
Distribute your weight: dont leave ALL of the patterns on 1 side of the room, this is a sure-fire way to make your space seem imbalanced. Rotate them throughout the room to even it all up. You could use a pattern on pillows,window treatments, a chair, ottoman, rug etc.
Dont mix your color saturations: Keep your hues on the same wavelength, mixing primary colors with pastels will probably not work; I'm not saying its impossible, but very tough to pull off
Consider all elements: If you have an oriental rug that you adore, mix fabrics that are not going to compete with this large scale pattern in the room. I would pick textured patterns, and more geometric shapes as opposed to large floral prints, medallions, and paisleys that try to steal all of the attention.
Think about the materials: I'm talking about the weight and feel of the textile here. Every fabric has a personality some are drama queens, some are sportier, others high society, some have a woodsy feeling, while others are like the southern debutante. I wouldn't go mixing a sporty fabric with the high society fabric. It would seem strange if you had an embroidered silk sitting next to a cotton calico print. Its like the people you surround yourself with, they all share commonalities with your personality and lifestyle, your textile selections should do the same.