Pressing (with an up and down motion) is very important when pressing seams and blocks. The pressing surface needs to be firm, not soft like a mattress because the fabric sinks in and is stretched out of shape. The right hardness of the pressing surface is a layer of COTTON batting covered with a layer of canvas or ironing board fabric. I learned this by experience and a video by Sharon Schambers tells you about it:
http://www.youtube.com/watch?v=4LGbXou_u4c
THis will help in addition to the other suggestions that have been offered.
Good luck, accuracy comes with experience!!