I do the following and it seems to help me:
1) In a texture editor, select the left side of the texture (or the right, it doesn't matter)
2)Copy the selection you just selected
3)Paste the selection
4)Mirror the selection across the X axis
5)Place the mirrored section on the right edge (or left).
Do the same thing for the top and bottom edges, just be sure to mirror the selection about the Y axis, not the left.
I hope that helps!
-Student Pilot