Super elevation helps combat centripetal forces by raising the outer rail of a curve. The steeper the banking, the faster a train can take a sharp curve. Although not usually necessary for operations, recreating the look of a banked curve in model form is an impressive and realistic effect.
To create superelevation, insert small strips of styrene under the ties beneath the outer rail of the curve. You do not need one under every tie. Styrene comes in many thicknesses; .040 to .060 works well for a bank in HO scale. You're building a railroad not a race track.
It is important to work up to the full bank using progressively thicker strips through the easement section of the curve. Make the transition as smooth and gradual as possible. Start with .010 styrene for a few ties, then .015, etc. As with the lateral easements, the more gradual you can keep the transition to the superelevation, the fewer operating problems you're likely to encounter. Reverse the transition process on the other side of the curve.