1. Balance loud and neutral.
Choose one colorful, highly saturated pattern that you absolutely love and adore. Then, pick out the most neutral color within the pattern (white doesn't count) and find a monochromatic pattern of it. This makes the room come off pulled together without being full of solids.
2. Pick a theme.
Whether it's floral or aztec or turkish or chevron, stick to similar shapes in a similar color palette. The commonality between shapes will tie the patterns together.
Whether complimentary or monochromatic, you can't go wrong with picking patterns that have the same color palette in mind. Be sure to balance between big and bold, small and delicate, geometric and even floral. If they're within the same color palette, they'll go together.
Sometimes, there's no logical explanation for why something should be paired with something else. Similar to relationships (yep, here it comes), there's no real reason why it works... it just does. To potentially stumble into something fab, you have to take some risks, pull some randos and start mixing. If it looks great, awesome and if it doesn't, on to the next.
Try taking things from other rooms and pairing them together - you never know what you'll find. Go on try it. I dare you.