i use starch when i want a little stiffness (crispness) to my fabrics.
i use sizing when i just need a little more than what water provides to remove wrinkles.
mostly i just use water.
when pressing- pick the iron straight up- move to next location- lay it down- don't 'iron' back and forth on damp fabric= it can stretch and distort it.
sometimes if you have stretched your fabric you can re-wet it--hang it to dry (or toss it in the dryer) and re-press it.