I just watched this video about wearing prints and plaids together.  Some good examples in the video.  Some that I thought missed the mark.  How do you make sure you don't end up looking like this gal?

You do need some rules of thumb to pull this off.  Here's a simple way to combine patterns.  Have one color in common with both patterns and vary the scale.  Look at this man.  He has a plaid jacket and a patterned tie.  The jacket and the tie both have red and caramel colors.  The plaid is larger scale in relation to the pattern in the tie.  And he has pulled it all together with a solid caramel shirt.  I give him an A plus.