I always pin, pin and pin. If I look at it and THINK I need to pin more, then I will pin, pin and pin some more!
I also use my dining table (when available) to lay my quilt on. I clamp the bottom down once it is nice and smooth. Then lay the batting on top, smooth that out and clamp it, then lay the top on and will clamp that. Then, pin, pin, pin.
Most of the time I use my sewing table which is a 6 ft banquet table, but it's getting harder and harder on my back since I can only reach from one side.