Adding a desaturated clouds layer above a scene will apply a dappled light effect, which can help liven up a scene, make it more natural and bring it all together. (Using Composite Photography to Create a Fantasy World by Karen Alsop).
Textures are best applied over a blurred background with the main subject sharp, and usually masked out of the texture. (2lilowls).
Textures can be added to Ps Libraries, which makes them easy to find and use.
Overlays & Textures to Buy or Download
Jerry Jones (**) - Wonderful free art textures and overlays