It really depends on what you're making. In almost any case you'll want to lay a nice base layer of color down. Even brown for wood, grey for stone and some metals, green for grass, etc. A noise filter over this base layer is a nice way to add some easy texture - just pay attention for color changes in some of the filters.
For wood (assuming we're talking cut wood, like planks), I then draw roughly parallel, random, thin lines (click > [hold]shift+click is easy) and adjust the color slightly (lighter, darker, browner, oranger, etc) each time or so. Dodge/Burn lines are good too. I run a filter (Noise > Spread)to randomize the pixels just a little, and then blur (Blur > Blur or Gaussian Blur) it just a little.
This will leave you with some nice soft textured stripes, but their still a little too regular in the vertical pattern to be good enough. This can be solved with the Clone Stamp tool. Be sure to have a very wide soft feathered edge on your brush for this. What you do is pick a different spot on the pattern (say between two stripes somewhere else) and clone a small bit of that vertical section (between 1/4 and 1/2) somewhere over the top of another stripe or area. The most important thing is to never use the clone stamp horizontally. What I mean is to always offset the area you are cloning TO at least a bit up or down (to avoid obvious patterning). Do this all over the texture, and don't be afraid to clone FROM areas you have already cloned to!
A little bit of this going back and forth and you have a nice randomly striped wood-like pattern. It's particularly good for an RTS like Glest because it doesn't have to be extremely high-resolution which means you can cheat like this (because it won't be photorealistic. This is the method I used for the wood planks (not including the rectangular ends, I had to do some tiling tricks for that) in the image below.
You'll notice that each plank has defined edges. For that all you have to do is burn some even lines across your wood pattern. After that a few thin lines of black created some simple straps to show that they were being held down/together.
The crates don't count because they were just a base color and some mild burns for the surface shapes of the cross-braces. A hack really
The metal of the vessel is grey and noise and burn shape. It was a work in progress and that's the part that was never finished.
The metal sheets were base color (grey) combined with a layer of Render > Solid Noise. Following that I used both the Airbrush and Burn tools to create the individual sheets as seen from side on.
If you want to see the texture file from the above picture, the TGA and XCF (gimp format) files are in the .zip you can get from the following thread.
https://forum.megaglest.org/index.php?topic=4587.0Hopefully this helps someone. I'll have to write up a proper tutorial with images too.
