One of the cool things about Blender (besides being free) is that it has some pretty good tools built right in for texture painting and texture sculpting.  I don't claim to be a master at this, but I've found a couple of videos that have helped me along.  These are all YouTube videos.

This first one is a "quick tip" that is actually almost 40 min long.  It covers various types of texture painting in Blender, some important settings (like Bleed), using images as stamps and so forth, and then goes into using images and textures in mesh sculpting.  Lot of very useful info.

Using Brush Textures in Blender 2.67+

This second one covers more on texture painting as well as guiding you through a complete project creating a low poly brick wall.  It covers one other useful option in Blender, baking a high poly model/texture to a low poly mesh. 

Creating a Grungy Brick Wall in Blender

And this last video is not a tutorial, its just a time elapsed recording of an artist's work flow as he paints a game sword in Blender.  Pretty cool results and though it wasn't intended as a teaching video I still picked up a couple of tricks just watching it.

Hand Painted Sword Texture in Blender