Lots and lots of plants (real or fake) to create hiding spots and at least two little caves/hidey holes. Watch your girls for a few days, maybe cup them and put them next to each other and watch to see who flares the most and who acts the most aggressive. When it comes time to put your girls in, cup them and float them in the tank next to each other for a few minutes if you can. Then, release the LEAST aggressive girl first and give her several minutes to explore. Keep releasing the next least aggressive girl at several minute intervals until you've let out the most aggressive girl last. Watch them carefully. They'll chase and nip and maybe tail slap. But if you get a pair who won't stop fighting after a few minutes, remove them. Wait a while, then release the least aggressive of the two.
The trick is to make the most aggressive girl the odd girl out so she doesn't have the ego to be a bully. It takes some patience but you'll get it to work. And if you're really lucky, your girls will barely fight at all.
like everyone said, lots and lots and lots of plants, the tiger lotus that is sold at petco is pretty nice, but has to be trimmed a lot so it doesn't take over the surface of the tank, you can add anubias underneath since it doesn't like bright light and also java moss or Marimo moss balls. Amazon swords are cool too, but with the tiger lotus it won't get very big leaves, at least mine don't. Java grass is nice looking. Horn root is awesome and eats up ammonia, but it drops needles all the time.
Having girls that are young and related, or at least used to living in the same tank, is also a good way to start. Mine were living in a tank at the petshop and travelled home in the same bag (not a good idea, but it worked for me). I've not had any fighting with my girls, even after adding a tiny little unrelated shrimp of a girl, less than half the size of the big ones. I believe that's fairly exceptional, though.
Java fern, rotala, ludwigia and lacefern are all great plants, too. Using driftwood with plants on top of it can help give you extra levels.
Also in my sorority I have Endler's Livebearers (tiny cousins of the guppy, extremely cute and colourful - the females are bigger and less colourful), ember tetras (smaller, slower and more peaceful than neons) and cories. My girls have had no problems with any of these.