Do it very slowly - starting with only a couple of steps, then gradually build up. If she veres to the right try practicing with a wall / fence on her right so she can't.
Put a crop across your hands so your hands are level and of equal contact.
Only allow your horse to step back as someone said a few paces before he gets chance to go all skew wiffy.
Only ask once, if he does it right, don't ask again...this is how they learn.
Practice in hand, I back my horse up along a long stable block sometimes instead of turning him round, i does this on purpose, it teaches him to back up on command and think about wear he's swinging his back end and I supose teaches him straightness..daft as it sounds!