Whenever possible, use adjustment layers to make your edits. Adjustment layers can be applied to one layer in the image or to any number of the layers below the adjustment layer in the layer stack. Adjustment layers let you to return later on and edit the amount of the adjustment or remove it entirely. So an adjustment made using an adjustment layer is removable and editable rather than permanently applied to the image.
To add an adjustment layer, choose Layer > New Adjustment Layer and then choose one of the 12 adjustments that can be performed using an adjustment layer. Those that are most likely to be of value are the Levels, Curves, Color Balance, Brightness/Contrast, and Hue/Saturation adjustment layers.
To apply an adjustment layer to only one layer, click the layer to apply it to and create the adjustment layer. When you're done, create a clipping mask so that the adjustment layer applies only to the layer below it. To do this, position your mouse pointer over the border between the adjustment layer and the layer to apply it to, hold the Alt key so you see the overlapping circles, and click once. The layer will be indented to the right in the layer list, and the adjustment will apply to this layer alone and not all the layers below it. To undo the clipping mask, repeat the Alt-click process.
When working with layers, keep a copy of the original image on the bottom or background layer of the image so you have the original image whenever you need to "borrow" a bit of it at a later date. To do this, when starting to edit the image, select the background layer, right-click, and choose Duplicate to create a duplicate of this layer as the starting point for working. If you need to use the background layer at a later date, make another duplicate of it and use the duplicate in your work. That way, you'll still retain the original image as a reference point in the background layer. Disable its visibility so its presence won't affect your edits.
When you encounter a technique that requires you to work on the entire image rather than a layer (sharpening, for example), it's tempting to flatten the image or merge all the visible layers to create the single image layer to use. Unfortunately, this destroys all the layer data in the image and means that you can't make any edits to it in future. Either make a duplicate of the image before flattening it or create a single consolidated layer in your image. To do this, move to the top layer of the image and press Ctl+Alt+Shift+E to make a layer that displays the entire image on it—as if you had flattened it or merged visible layers. When you do this, notice that the existing image layers remain intact and editable. You can now apply a filter or another technique to the newly consolidated top layer without affecting the original data below.
When you're working with layers, name each layer with the layer contents and fix so that they can be easily identified later on.