Try Not To Correct More Than Once
When correcting images in Curves, Levels, etc., it's best to try to 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.
Why not use a curves adjustment layer so you don’t run into this issue in the first place? The curves adjustment layer does no damage to the image so you can adjust as whim then save out your final adjustment. I will typically create a curves adjustment layer, then merge my new image on a new layer and continue working. If I need to make further curve adjustments I’ll just go back to my non adjusted layer and go from there…
Adjustment curves all the way. They’ve been around since CS2