try spray starching the material, don't iron, just press, matching up seams, I usually iron the seams in oposite directions on each piece so that when joining, they will nest...sometimes I will also stick a pin in the seams then pin on either side of the seam, remove middle pin and sew.
hope this helps. Also I learnt this from Eleanor Burns, find you 1/4" seam, I then put a piece of tape where my fabric will run next to, then I move my needle over one position. this gives me a scant 1/4"....