Tip: wavelet decompose : a good cure for tiling patterns.
When making tiled textures you must avoid big repeating patterns , that will ruin the look of a texture from the distance.
Here’s a good article about the use of highpass filter : http://www.blendernation.com/2007/04/13/the-power-of-the-high-pass-filter-for-texturing/
The idea is that you can extract the “high frequency detail” .. that is small patterns , which will look good and with enough variations when tiled.
That also means getting rid of big shadows and gradients that you don’t need in texture .
Following the same concept , you can use wavelet decompose (as a gimp plugin) : http://registry.gimp.org/node/11742
You’ll get various layers , each containing different sizes of detail , stacked to compose the original image .
Then you can easily find a layer with the size of detail you want to enhance (unsharp mask , contrast) or remove (simply erasing parts) .
I find that a very efficient way of both sharpening the map and removing low frequency noise (big patterns and shadows).