Proper pressing is a prerequisite for accurate piecing. Press with a light touch, using iron tip and an "up and down" movement. Save continuous motion "ironing" for wrinkled fabric. Use either steam or dry heat, whichever works best, and assembly-type pressing to save time.
Choose a pressing plan before beginning a project and stay consistent, if possible. Seams are "set" by first being pressed flat and then pressed either to one side, usually toward the darker fabric, or open. Sometimes, both are used in the same project, depending on the design.
To prevent distortion, press long, sewn strips widthwise and avoid raw bias edges. Other pressing hints are: use distilled water, avoid a too-hot iron which will cause fabric shininess, and pre-treat wrinkled or limp fabric with a liberal amount of spray fabric starch.