TRY NOT TO CORRECT MORE THAN ONCE

When correcting images in Curves, Levels, etc., it's best to try and do all your corrections at one time rather than changing each setting individually (by that I mean, don't set a highlight in Curves, then close and reopen it to set a shadow). The reason is, each time you apply a tonal correction, it puts some strain on the quality of the image. So to keep your image from having unnecessary data loss, when you open Curves or Levels, make your shadow, highlight, and midtone adjustments, and then click OK to apply all three adjustments at once.