When you are attaching the binding and come to the corner make sure you stop 1/4" (or whatever seam allowance you are using) away from the end. Don't eyeball it, get a ruler, measure the seam allowance you are actually using, then mark where to stop. It's better to stop one stitch too short than one stitch too far. When you do your fold, make sure that it's 45 degrees. Again, if you are not sure, use a triangle ruler. Then when you fold the binding back on itself to go down the next side, make sure the fold is even with the quilt. A teeny tiny bit hanging over the edge is OK, but if you have a lot hanging over (an 1/8" is too much), or it's too short your corner won't come out. After you have attached the binding and are turning it there are also a couple things you can do. First, try snipping out the batting in the corner. Don't snip the fabric or the thread, just the tiniest triangle of batting. This may allow the corner to lay better. When you fold the binding over the corner & do the miter, you have 2 ways to fold it. Try both. It will often be easier one way than the other.