It's easy if you just want to make it hurt, not competitions or anything..
easy this one. I did a couple of self defence classes.. basically with your weaker arm, you deflect the punch, then with your strong hand(Right in my case) you do the palm push type thing..
eg:
Like that for hand positiion(Sorry about pic

)
Just extend your wrist as fast as possible so the guys nose lands in the centre of your palm.
Has to be a forwards and up motion.
Apparently a broken nose discourages people
edit: this illustrates it better: