[bad beetz]

Easy.

When you're squared off with the dude with the knife, remove your shirt.

This will let him know that you are seriously getting ready to throw down. No one stabs you in a ring like this if you're "getting ready"

Next... and this is the key part, remove your shoes. This makes it looks like your some martial arts master, and buys you more time

finally, remove your pants and underwear. No one will stab a naked dude, it's just too weird. People will probably just slowly turn away.