Duplicate, duplicate, duplicate. Make copies of your layers after each successful stage. It can be frustrating to get near the end and find there was a mistake early on in the process–but if you have an earlier version to return to, you can correct your errors far more easily.
Name each layer as you create it. If you use a filter, consider naming it with the settings you used – such as “Unsharp Mask, 2, 150, 0”–so you know how the effect was achieved.
Always experiment on a copy. Photoshop is ideal for tinkering and trying out new ideas–but make sure you keep a copy of the original before you start down an unknown path.
Be creative with filters. The Plastic Wrap filter doesn’t just wrap objects in plastic, it can be used to create liquids of all sorts. The Clouds filter may produce lousy clouds, but it’s a great random texture generator. And give the Wave filter another chance, it’s better than it looks.
Don’t erase anything. Use a Layer Mask instead. That way, you can always reveal pat of a layer you’d previously hidden. Once it’s erased, it’s gone.
Rather than applying a Curves or Color Balance adjustment to a layer, use an Adjustment Layer instead. The effect will be the same, except that we can go back and change the adjustment at any time–or copy it to a new layer.
Learn to use the Pen tool. It’s the single scariest Photoshop tool, and many users just give up on it. Take a day to master it and you’ll value it for the rest of your life.
Don’t forget the shadows. Shadows on objects, shadows beneath objects, shadows on the wall behind objects. Once the composition is finished, it’s the shadows that really bring it to life.
Convert layers to Smart Objects in complex compositions. Each time an object is scaled, rotated or distorted, some quality is lost. With Smart Objects, we can tinker as much as we like without losing any quality. It can be heartbreaking to see an image looking soft or ragged, simply because we changed our minds one time too many.
Similarly, convert layers to Smart Filters, you can now apply Gaussian Blur or Unsharp Mask, or any of Photoshop’s many other filters, and go back at any time and change the settings we’ve applied without harming the image quality in any way!