Tip 1: Use adjustment Layers
(Click for a full-res image)
These sit on top of normal layers and affect all the layers below. They don’t modify your pixels directly, so you can always go back and readjust, and you can add lots of them with only the barest increase in file size. Each comes with a handy premade mask. To add one to your image, go to Layer > New Adjustment Layer and choose the type you want. You can adjust Levels, Curves, and more, or add a Photo Filter, as I did here.