Are you using a walking foot? Free motion quilting?
To me, this sounds more like a problem that is happening at the machine than something you are doing in the basting stage -- though I do agree that tape helps when basting.
A little trick that can help when you have persistant puckering is to machine baste the quilt in a grid with water soluable thread. This keeps all the layers together and smooth while you go in and sew your more interesting quilting design, but will wash out easily.
RST