1. Put the two (or three or four) patterns next to each other.
2. Imagine that the fabrics are all the part of one garment.
3. Would you buy it? Would you wear it?
If so, go for it. If not, try another mix.
Some of my favorite pattern mixes:
This selection illustrates some of my pattern mixing workhorses: the obi belts, striped cardigans, and anything floral or animal print. You can see the rest of my pattern mixes by clicking on the Pattern Mix tag. Some combinations are subtle (patterned tights with a checked dress, perhaps) and some are pretty loud (like anything I mix with that zebra dress).
How do you mix your patterns?