You know how you need to prewash most fabric to keep your finished project from being a shrunken, lumpy mess? If you just toss it in the wash, though, it comes out a stringy, tangled mess.
If you have a serger, serge the cut edges before washing. I usually just leave the thread tails long and they don’t unravel enough to be annoying.
With a sewing machine, you can sew a quick zig-zag or similar stitch along the cut edges to prevent fraying. Even a straight stitch would probably work, although I haven’t tried it. You will probably need to back-tack or knot the ends to keep it secure through the wash.
This also lets me see at a glance which fabric was prewashed by looking at the edges.
And I know 90% of the people reading this are thinking “Duh!” because it’s such an obvious fix. The other 10% are wondering why they didn’t think of that, much like myself when I first learned the trick.