When practicing 45-degree banked turns for your flight review or a practical test, where do you tend to focus most of your attention? On the attitude indicator? The altimeter? The vertical speed indicator?
If you answered yes to any of these, you’re looking in the wrong place. Your primary attitude reference during steep turns (and lots of other flight maneuvers) should be the outside horizon. Sure, you’ll want to glance at the attitude indicator, VSI, altimeter and heading indicator during the turn to confirm you have the proper amount of bank dialed in and aren’t climbing or descending, but you should be able to complete this maneuver with very little altitude deviation even with the altimeter covered.
In fact, having your flight instructor cover the altimeter and seeing how well you do by focusing your attention outside the airplane is an excellent way to make the connection between your eyes and your brain of precisely what the sight picture out the windshield should be. Once you’ve gotten fairly good at performing steep turns with the altimeter covered, try them again with the instruments uncovered, but continue to focus most of your attention on the scene outside the airplane.
Before long, you’ll instinctively know whether what you’re seeing outside looks right and what you need to do to correct. Once you’re comfortable making steep turns by outside visual reference, you’ll automatically find yourself relying less and less on the instruments in other phases of flight, and that will make you a better pilot.