I don't know about you but I love making patterns, but I've always had a hard time figuring the repeat out. Here's a great tutorial that works in every edition of illustrator.
Hat tip to Daniel Fishel for showing it to me!
Plus, if you find you're getting little white lines in your patterns that drive you nuts, here's a simple tutorial on fixing that!
For me, what fixed it was in my Illustrator preferences I turned anti-aliasing off and then on again and they disappeared.