I snip into the selvage on one side (close to the cut edge), then rip the cut edge off. You end up with a little bit of a lasagna edge, but at least it's straight. From there, you can work at pressing the fabric straight and flat. And selvage edges should lay on top of each other when the fabric is folded in half lengthwise and the ripped edge is on top of itself.
Sometimes, easier said then done, I know. But that straight edge is true, and dampened fabric can be manipulated to be straight, then dry ironed. Well worth the extra time. That first straight edge, gets you off to a great start.