Mirroring is the ease of simplicity and works with any image equally well, creating unexpected patterns.
Choose your image that you want to tile and copy it to Clipboard.
Now resize the image window 200%, making it twice as large, and clear the window so it is blank.
Paste the original from Clipboard; it will take only one-quarter of the enlarged window.
Flip the window horizontally and paste the image from Clipboard again. You will see the top half of the window mirroring itself.
Now copy the entire window to Clipboard, then flip it the window vertically.
Paste from Clipboard and you have a seamless, tileable image. You can then resize the image to 50% (back to original size) or whatever works best.

These directions are demonstrated in the making of a Scrib.

Select this image to see its tiled appearance.