I lay the batting on top of my large table and, working in sections, thinly spread a glue that washes away on the batting, and then smooth the backing on top of it. After the back is done, I flip the batting over, and do the top side. If the top has a eon of seams, I pin it down. Those seams can cause shifting too as the quilting foot might move differently over heavy seams.
I often make the blocks on the back of the quilt the same size as the blocks on the top. I like to work in 12" blocks. That way I can match the corners of the backing block to the corners of the top block and pin at the intersection of those corners. This can be tricky, so I take my time.