Remember the rule: "If wearing a solid shirt then wear a pattern tie" and vise versa? Well, that rule and the idea behind it are meant to be broken! I used to have this mindset when I started wearing dress shirts and ties. I'd wear a striped shirt with a solid tie or solid shirt with a pattern time. I'd never mix patterns or colors. Eventually, I got bored and started experimenting and pushing the
norm (what could I get away with..hmm). I've grown to love mixing patterns together. It's just another way to express yourself,
stand out and be different. 
A rule to keep -->> never wear patterns on top and bottom, unless you have the body type to pull it off (usually taller and leaner). If I'm going crazy on top with mixing and layering I try to keep my pants simple, then add a pop of color/pattern with my socks and style with nice shoes.
Mixing patterns has taken off this season, so this is the time to take risks in this area if you haven't. Warning -->>There are times when being simple is a better choice, say at a job interview or an important meeting (depending on the type of work). When this happens you can still mix patterns, simply choose your colors differently (like blues, blacks, grey, dark green). Your boss may not approve of you wearing hot pink! Ha! Know your environment.
Other than that I'd encourage you to go ahead and mix polka dots with plaid, gingham and stripes, paisley and checker board, the combinations are limitless. I have added some of my favorite pattern combinations in my blog, but I have worn many more. Almost all my outfits include some amount of mixing and matching, it would be impossible to share them all, so check them out on instagram (Username: newman03). This is just another way to be creative and have a little more fun with your wardrobe. 