Sunni, Besides all the helpful hints above consider trying glue basting, discussed today in this thread
http://www.quiltingboard.com/t-158048-1.htm It would mean a few minutes extra getting ready to sew your rows, but perhaps many minutes less than ripping them out or being unhappy with the oopsies! I haven't tried the glue yet myself, but I do pin every intersection thoroughly; that helps a lot but has not kept slippage entirely at bay, so I'm hoping the walking foot I just bought will be the final solution.