Pin, pin pin! Sometimes your seams will go together without pinning, but most times, you'll have to pin. But I find that not actually pinning the seams, but pinning OVER the seams, helps keep it straight . When you put a pin in the sem allowance and then do that little motion to make the point come back out again, it WILL throw the seam out of alignment. I pin about 1/4" before the seam and 1/4" after the seam. Also those 2 pronged pins work very well for pinning on each side of the seam.