I know it's not very exciting, but you have to take your time. I also set my seam, then press from the front. I tend to assembly line sew, so then I assembly line press. I figure out which way I want the seam to be pressed, and lay out a row of sewn units with the fabric that the seam will be pressed toward on the top. I set the seam on all the units, then I flip the top fabric over and lightly finger press the first unit (so I'm pressing from the front, while the fabric is still a bit warm). I put the iron down on that unit & finger press the next one, then move the iron to that one, moving across the units that I have laid out. This allows the iron to just sit on the seam for a few seconds while I'm finger pressing the next one. I try & let the units cool before moving them from the board. I pre wash, often starch & usually use steam, but as a previous poster mentioned, let the iron do the work for you. Heavier is better, so even if I don't use steam, I make sure the iron is filled with water.
I do the same for joining rows, moving across the row: set the seam, all across the length of the board, flip to the right side, finger press a few inches (about the length of the iron), set the iron down, finger press the next section etc.
Usually this results in flat seams. Sometimes a fabric doesn't want to play nicely, so I do use a clapper. I also have a Steady Betty which I really like for complex blocks with lots of bias.
"Let the iron do the work for you" was one of my mother's mantra's, another was "let the item cool before you move it"