Warp separator is the thing that keeps your warp threads from slipping between each other. The reason you need this is because when threads fall between each other, they go around the warp beam at different diameters. Threads going around the warp beam at different diameters has an adverse effect on tension.
Any manner of material may be used, but whatever you use, you want it to be something stiff yet bendable (so it can go around the beam with the warp) and also something that won't compress (as in corregated cardboard which tends to compress). Freezer paper, heavy brown paper, card stock, venetian blind slats, etc., or my favorite: match stick blinds. There are special "warping sticks" made for the purpose that you can purchase or you can make your own. If you're the handy type, lath comes in bundles of 50 at the hardware store. You can cut them down and sand them and they make wonderful warping sticks.
Laura Fry has an excellent post about her experiences, over the years, regarding this topic. Ever since I learned about this, I have been using match stick blinds and they are, in my opinion, the best ever things to use. I realize that not everyone may like them and we all have our favorites, but I've tried all manner of materials and the match stick blinds win, hands down.
Now, on to Laura's post. Enjoy!