Don't fire back. Firing a comment back will just make the situation worse as now you have created an argument as the mean person is NOT going to back down. If you don't fire back and just walk away you might be surprised with the outcome.
Walk away. The person being mean might yell at you as you leave saying things like "wuss" or "come back here" but no matter how much you want to yell back at them, don't this will make the situation worse just continue on your way and leave the mean person talking to themselves because YOU DONT CARE.
Be nice. It's really hard to be mean to a nice person, try it. The next time someones mean to you say something nice to them like "I like your clothes style" or maybe "your pretty" be nice as it is usually hard to be mean to a nice person.
Find an excuse to leave. If you have no way of doing any of the tips above find an excuse to leave, any reason to get out of the situation.
Avoid the person or get support. Avoid the person being mean at all costs and maybe they will stop being mean.
Ignore. This is your last option, if all else fails ignore them.

Why be mean?

Why mean