There are some automated plug-ins for this that will cut and blend the source image to make it tilable in horizontal or vertical direction, or both, or in a "diamond" shape (kinda like 45 degrees rotated squares). These work for some textures.

For textures where this doesnt work satisfactory you would have to cut it up in 4 quarters and put them back together so that the 4 corners of the original image end up in the center of your image.
Lets call the original photo version TextureA, and the cut up version TextureB.
So if the quarters were originally located as:
A B
C D
in TextureA, they should now be organized as:
D C
B A
in TextureB.

Once you've done that you usually use the clonebrush in photoshop to try to adjust TextureB in such a way that the 1st clearly visible edge is no longer notisable.
If you then really want to make it clean you would have to cut up TextureB again to make sure the visible edges you cleaned up match when the left side of your new TextureB image would be put against the right side, or the top would be put against the bottom.

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