the technique is really simple: when you've pressed your block, set your floppy book down on top of it and leave while your block cools, 30 - 60 seconds. usually, i lay out the blocks or pieces i'm pressing in a row across my pressing board, and move from one end to another, covering up the pressed blocks as i go. i have a couple of phone books on hand, which makes doing batches easier. usually by the time i've gotten to the end of the row, the first blocks are cool. i can move them out of the way and do another batch, rotating out blocks down the row.
letting the blocks cool completely (or nearly completely) before you move them is key to getting the fabric to retain the shape you want. while hot, the fibers will continue to change shape when moved, so let them alone until they cool.
sometimes, when i have an extra stubborn block with lots of seams, i'll even leave it sitting under the book overnight after i'm done sewing for the day.
also, i prefer to do this before i trim my blocks. i find they hold the final shape much better if i've let them get properly flattened out first.
|lecien sugar flower collection circa 2010|
i had a stack i'd used the books on and a pile that hadn't been flattened. the book pile was definitely flatter across the board. once i saw the difference, i went right ahead and flattened all the blocks before thinking to take a photo to share. but trust me, it helps a lot.