How do you square up your quilt after you have quilted it? After spending lots of time making sure my quilt is square when I am piecing once I quilt it I find it has pulled up in areas and is no longer square. Before I put on my binding I wet my quilt then lay it out on my tile floor. I use a couple of tools including the tiles to get it straight. I use a laser beam to shoot from corner to corner and gently shape my very wet quilt. Next I check my corners and borders with a large square ruler making sure everything is lined up and adjust where needed. Finally, I will mark any new lines, with chalk, that I need to use when putting on my binding.
If your quilt is not square no matter what you do it will not hang flat on your wall or worse yet at a show. Another other cause of your quilt not hanging straight is uneven quilting. If you heavily quilt one section of your quilt, but your quilting stitches are far apart in another area this will cause a wave effect.
One of my pet peeves is when piecers add borders without measuring. You should measure through the center of your quilt to find the length of your borders. The borders on opposite sides should be the same length. If you simply add borders then chop off the extra length there is no telling what length they are. A 36 inch border on the opposite side of a 36 1/2 inch border does not make a square quilt.
Finally, take care to hang your sleeve on straight. A slanting sleeve makes for a slanting quilt.
I hope these tips were helpful. What tips do you have for squaring up your quilt?