My secret -- Elmer's washable school glue. Whether I nest seams or iron them open, when it's important that they match perfectly I glue baste before sewing. To do this I go to the ironing board, put a pin straight down through the exact intersection I want (the pin is a scant 1/4" in from the cut edges, where the sewing line will go), lift up the seam allowance and put a tiny dot of the Elmer's washable school glue in there, then press with a hot iron for a few seconds. This dries the glue. I take the pin out and then open up the fabric to make sure that the seams are exactly where I want them. Only after all these matches have been done do I go to the sewing machine and sew the seam.
Edit: I should mention that there is enough padding on my ironing board (it is a layer of Warm and Natural covered with cloth) so that I can put a pin straight down into it.