It seems counter-intuitive, but if you sew your seam, stopping just shy of the corner (needle down position), then sew one or two stitches diagonally, stop again with needle down, and turn to sew the other seam, you will find that, after trimming the excess, you will get a much sharper corner. This works well with bulky fabrics, but also with lighter weight fabrics, too. Try this with collars, you will like the results.

I think these turned out very nice, even though you may think that the rounded corners don't look as nice. I bet everyone loved them - we sewers are often too critical of our own work, and notice things no one else does!