Leave visual "breathing room" between two patterns. Use a solid fabric between two different bold patterns. In this photo the white euro sham pillows are use as an important visual transition to the color and patterns of the throw pillows and duvet cover.
Work with three different scaled fabrics.Find one large pattern, one medium, and one small patterned fabric. If you were to use the same sized patterns throughout your bedding set, it would look way too busy. Consider scale and size of your textile designs and vary them to create visual interest. In this picture, the large zebra print on the euro shams contrast well with the circle patterns on the duvet cover.
Mix in patterns through color continuity. Choose different patterns that share one same color, and another that is similar to tie it all in. For this space the blue is the dominant color, and black is an accent used sporadically to tie in the color scheme.
Consider the color intensity of the patterned fabric. Use similarly strength hues like warm colors together, and bright colors together. A pastel patterned fabric and warm colored patterned fabric may not go well together.
Mix two different pattern scales. For example a big bold floral pattern with a geometric pattern in this photo, that also matches the artwork above the headboards. Again, take note of the pillows, as a visual transition between the patterns.
Make a black and white bedroom look interesting by adding a number of different patterns.
Choose one dominant pattern to highlight, and mix in two other patterns, giving them smaller parts, like in this bedroom setting.
Try pattern layering using different geometric shapes. Concentrate solely on geometric patterns in varying sizes.