When piecing fabrics, it is essential to press each seam as it is stitched. Firstly, press the seam flat on the wrong side before opening the pieces, to set the stitches.
Then, on right side of the fabric, press both seams towards the darker fabric using tip of the iron and then press again on the wrong side.
Seams can also be pressed open as this helps to distribute the bulk of fabric created when lots of seams meet at one point.
Press rather than iron, preferably without steam. Spray starch can also be used. To give a final polish to your seam, after pressing and whilst still hot, place seam under a heavy book for a few minutes until cold. This will allow the natural fibres to set in the same way a hairdresser sets your hair with a cold shot of air.
In a Nutshell, can be seen in full in British Patchwork and Quilting magazine.