Make sure there is more than one feeder (set on opposite ends if you have an enclosed run) available is a good start. That way the low girls can have a chance to eat without getting beat up.
If it's just one chicken doing the bullying, you can isolate the bully for a few days (in a dog crate or something w/food/water). Sometimes that brings that bird down on the pecking order.
There's also something called pinless peepers, that are kind of like blinders for horses...google them or do a search here on BYC.
Over a weekend, you can sit out in the run w/a spray bottle and a book and spray the meanies (this helped quite a bit with one of my bullies) when they go after the weaker one(s).
Also, if your birds are contained in a run, make sure there's plenty of roosts, stumps, things to do and barriers...