I'll join you. This is a very basic tip about adjustment layers, but I was ignorant of it for years. When I used to tweak pics in Photoshop, I would go to the Image menu, then pick adjustments, then tweak brightness/contrast, hue/saturation, color balance, levels, etc. This was all done on the image itself without making new layers. (Sometimes I might copy the background layer, just to keep it preserved in case my changes screwed it up too much.)
I have used Photoshop for years, but never understood the value of adjustment layers and layer masks. For those who don't know what I'm talking about, many of the tweaks you might make on your pictures (levels, brightness/contrast, hue/saturation, color balance, etc.) can be done in individual layers. Those are called adjustment layers, and each adjustment layer has the added benefit of a layer mask that goes with it.
To create an adjustment layer, go to the Layer menu, then choose New Adjustment Layer, then choose the specific tweak you want to make. Alternatively, if your layers window is open, just click on the white/black circle in the middle of the items on the bottom of the menu. It will give you a list of tweaks to choose from.
Say you want your subject to have normal color saturation, but you want the background to be desaturated. I used to go through a laborious selection process so that an effect might affect only part of the image. With adjustment layers, however, you choose the hue/saturation adjustment layer, and that new layer appears with the hue/saturation control box open and ready. You reduce the saturation so the background looks right, then close the control box.
Wait, your subject is also desaturated? Here's where the layer mask comes in. The layer mask is the all-white box on the same layer in the layer window. All parts on the mask that are white let the desaturation effect go through. Click on that white box, then get a paintbrush with soft edges and choose black as your color. Now start painting the black directly on your subject. If you have properly selected the layer mask, you will not see black painted on your subject. Instead, you will start to see the original saturation levels return to the places you are painting black. That's because the black paint is on the layer mask itself, and the black blocks the layer's effect. If you paint with lower opacity (shades of gray), you can control the amount of the effect you let through.
One other great benefit of the layer mask is that if you overdo the effect, you can then go to that layer's properties and turn down the layer's opacity, thus reducing the effect over the entire picture.
If you don't know about adjustment layers and layer masks, there are many videos on Youtube that show you how to use them. Much more info than I can put here, but it's the best new thing I've learned in PS in years.