I read a trick that works for me...Take a square piece of paper and hold it up in front of the mirror. Our eye can't see the stretch in irregular objects but can identify when a proportional, square shape warps into a rectangle. Sometimes it's pretty shocking to see how far out of square it is.
My other, less technical, trick is to look at how I look in the clothes I wore into the store compared to how I know I look. I check myself out in my street clothes as soon as I get in the dressing room, then try on clothes, then put my street clothes back on and compare how I feel I look in them compared to the tried on clothes. If the clothes I tried on aren't better looking in the store mirror than what I wore from home, I pass. This system works pretty well for me.
