It is important to pay attention to your hand entry during backstroke.  Think about having a huge clock encircling your entire body.

Imagine your head position is at 12:00.  The goal should be for your hands to enter at 11:00 and 1:00. If you tend to cross over or enter your hands too close to 12:00, you may want to practice drills entering at 10:00 and 2:00.

Below shows hands entering too close to the head causing crossover:

Backstroke 12:00 crossing over from Marli McIntire on Vimeo.

The following shows a better hand entry placement: 

Better hands entry backstroke from Marli McIntire on Vimeo.

During your next backstroke set, ask your coach if your hands are entering in a good position.