Here's how do it:
First I press to relax the seam.
Then I use my fingertips to open the seam "walking" them along the seam.
Then I use my fingernails to "press" the seam open from end to end and back.
Last I press right to left using my left index finger to keep the seam open as I carefully left the iron up and over only an inch or so: keep your left index finger at least half an inch away from the tip of the iron. Press slowly!
Sounds like it takes a lot of time but once you get in the groove of pressing seams open, it doesn't take any more time than pressing them to the side.