We use a mirror attached to the wall near the heat press. I lay the shirt out flat on the table, stick the heat press on, then hold ths shirt up in front of the mirror located about 10 feet away to make sure it is straight before pressing. Works great for us.
Look at your shirt. Most have a fold crease in them. I made a line down the center of my press pad and the align it with the crease. Like Dizzy said. Just takes practice. There's no magic bullet.