Just some quick tips for when you're doing any digital painting, whether it's for a matte painting or a texture map, or anything else. If I think of any other tips that would fit in here, I'll update the list.
Use A Tablet
It's infinitely easier to paint with a wacom tablet than with a mouse. The ability to use pressure to control your brush size or opacity or any number of other things us crucial.
I usually have my color sampler (eyedropper) set to 3x3 pixel sampling. It's a great way to get an average color to blend between two colors. This is good for when you need to make softer edges or when creating the wrapping light around curved surfaces.
this is a much better way to sharpen your images and texture maps than just using the standard Sharpen filter.
Embossed Texture Overlay
Sometimes a good way to give something a textured quality is to use a photographic image that is run through the emboss filter, and set the layer to overlay. This is kinda similar to bump mapping in a 3d program.
Desaturate to Check Your Values
I don't often use this, and I probably should use it more, but it can sometimes help to desaturate your image so that you can more clearly see the luminance values of the image.
When objects get closer to each other, there tends to be bounce light. This is an almost subliminal way to bring realism to the picture.