1. Keep the color scheme similar, and maybe even throw in a solid that picks up on a color in both patterns and pulls the look together.
2. Some patterns just work better together - polka dots with stripes or florals; stripes with florals; animal prints with non animal patterns or paisley with squares or checks. Think of stripes as a neutral, they go with everything.
3. Don't try to match them like a small and large plaid. And try to keep the scale of the prints different so there's one large and one small.
4. Keep the rest of the look neutral.