I came up with a symbol for my allied armies. They aren't allies in game since my friends and I aren't fans of the ally system but they are allies in my fluff.
It's rebel-turned-mercenary Tau, Corsair Eldar, and some Rogue Traders. My friend came up with the name "Dark Star Alliance" and I really liked it. I initially made 3 symbols in Adobe Illustrator and went with the one on the right:
I print them on decal paper, seal them with clear acrylic spray, and just apply them like normal decals.
I bought this pack and it's enough to do more models then I ever plan on having:
http://www.decalpaper.com/product-p/tryc.htm