Start out with a stable aircraft. Get your speed where you want it, get on altitude and trim it up for hands-off flight. Once you're ready for your steep turn, I recommend rolling into the bank fairly quickly. Don't be timid and milk the bank in thinking that is going to help you maintain altitude. You don't have to go full aileron, but roll in so that you're at 45 degrees within 3 seconds (don't forget to lift the wing and clear first!). While you're passing about 30 degrees of bank, increase power 200 RPM and give the trim wheel 2 turns nose-up as rickair said above. When I was teaching, I used to show my students this "trick" and they were amazed when I took my hand off the yoke and the airplane flew +/- 100 feet by itself (if not +/- 50 feet!). When you stair step the bank and take too long getting to 45 degrees, you now have two things to worry about at the same time; pitch AND roll. You want to limit the amount of axis you've got your left hand trying to control at the same time and let the airplane (trim) do the work for you. The airplane flies just fine without you. You're just there to change its attitude every once and a while. This should get you pretty close to a nice, stable steep turn. From there, fine tune with power and trim.

My own personal liking was to have the plane trimmed *slightly* more nose-up than I needed because I felt more comfortable controlling altitude by putting a little forward pressure on the yoke than I did having the pull, but again, that was my technique.

Other words of wisdom: Don't obscess on the attitude indicator while rolling in and out of bank. Set what you think is 45 degrees by looking outside. It's not rocket-science. If you passed geometry class in middle school, I'd be willing to bet you can set your bank within +/- 5 degrees with your AI covered. It's pretty easy to see what 45 degrees is by looking outside. The most common mistake private pilots make with just about every manuever is LOOKING INSIDE TOO MUCH. For some reason I like using the altimeter as my primary pitch instrument while trying to maintain altitude. It gives me pretty much instant feedback on where my pitch is going and where it should be. I can see my rate of climb by how fast my altimeter needle is moving. I don't need a VSI for steep turns. If I'm shooting an ILS, that's a different story. There are hundreds of techniques, but your CFI should definitely be teaching you the basics and not telling you to "stay ahead of the airplane".

Good luck.