What I see in Valve's rock textures
-Start with a base. Use this more as a ref for shapes and shading.
-Use a ton of layers, one for brush strokes, etc
Start with a solid foreground color, use the dropper tool to pull the most prominent color from your ref
-Draw big organic shapes for rocks (I suggest the pen tool). This is just a guide layer, so no coloring happens here.
-In a diffrent layer, use a wide bristle brush on a high opacity(I don't know what brushes come with PSE, so you might want to download a few).
-Many of the rock textures that Valve has made use a fading gradient on their strokes to simulate weathering (with a tablet that would be pressure and sensitivity).
Wood----------
-Again, the almighty ref is used
-Start with a solid fore ground color
-GIMP comes with a filter that makes strips (grains in wood in this case), use this on a seperate layer. Of course, you could draw them yourself, but with a mouse it's diffcult.
-Gimp also has a swirl filter which can create imperfections on the wood, I belive PS has this as a tool.
-Create a new layer for dirt and chips.
-Dirt to Valve is a large rectangle which you then use a eraser on to show diffrences around it.
-the dirt also has a gradient on it that shows fading as i goes more towards the center.
Tile-----------
-Start with 4 squares. These are relevant to the texture's size (512 x 512 texture = 256 x 256 sized tiles.
put each tile in it's own layer and make each a diffrent color (for selection).
These are just pointers, tips, guidelines, whatever. Just go nuts on that tablet