Many people get intimidated and confused when it comes to mixing patterns and using more than one in the same room. But there's ne need to be. Coordinating and mixing patterns is actually quite easy.
Patterns don't have to match. I repeat, patterns do not have to match. Like with many elements of decorating they simply have to coordinate with and complement each other.
How to Mix Patterns Using Color
The easiest way to mix patterns is with color. Feel free to mix florals, plaids, modern motifs – whatever – as long as they share a common color palette. That doesn't mean they all have to be made up of the same colors, they just have to share some similar colors and tones. While this idea may seem intimidating to some people it needn't be. Learn to trust your eye. You'll instinctively know if something looks good to you or if it doesn't seem quite right. Trust yourself and your own sense of style. After all, it's your house so it has to look good to you.
Remember also that patterns don't have to be full of wild colors. Patterns made up of neutral colors and shades are great for adding interest to dull spaces. Imagine for instance a flat cream wallpaper with a glossy cream diamond pattern on it. The effect is very subtle yet adds visual interest and elegance. You can also use various shades of the same color if you want to play it safe.
Size and Scale
When using patterns it's important to think about size and scale. The best looking rooms have a mix. For example, try using a large pattern on a large area such as window curtains or on the walls; medium scale patterns on furniture; and small patterns on small items such as throw pillows. (These rules aren't set in stone, use your judgment based on the room, the pattern, and the place you want to use it.)
With all this said remember that too many patterns in the same room can be a bit overwhelming and contribute to visual clutter. Be sure to use a solid color (or a few solid colors) to create balance and pull all the patterns together.
When it comes to mixing patterns the most important thing to remember is to have fun and stay true to the looks you like best. When you do, chances are good that it will all come together without too much difficulty.