Originally Posted by Voodoo

First, make it tile / match up properly WITH a seam, just so it will match the lines etc... and everything looks right but might have a shadow seam.

Then divide your height by 2, and the same for your width. In this instance...
240x180 = 120x90

Go to Filter -> Other -> Offset
Horizontal: +120
Vertical: +90

This will give you a + crosshair across the middle of the image.

Use your paintbrush or stamp tool to patch the seam with samples from the rest of the image that match the area you are patching.

Next... Run Offset again and look at it. Sometimes there are some edge discrepencies that need a little more patching. Just clean those up. Run Offset one more time and it should tile properly.

That's about it.