Layer Masks, one of my favourite tricks is to duplicate a layer, make some changes to it, whether it be colour contrast b&w, or some other effect I want to, apply to the image then add a layer mask and brush in any differences I want to see applied to the image.
Using a black and white image on top of the other is one of the most common, but intensifying colour using curves or levels is one of my favourites. Again this approach in non-destructive, and infinitely un-doable, while the layers are unflattened.
Lastly the Text Layer, which is relatively self explanatory, I hope? Handy for adding a copyright watermark to your image I guess?
