there is at least one tutorial for making seamless textures in here.. .basicly you take your texture - cut it in half vertical shift the halfes so the outer borders are now in the middle - then you merge them again - then you cut it in half horizontally and shift the halfs again and merge again - so now all your outer borders are seamless cause they represent what used to be the middle. Then you fix the new middle with healing brushes, stamp tools etc to make it look good (remember not to get to close to the new edges) and Voila! a seamless texture