|Take the image you want to make into a tile. Select 'all' and copy (but don't paste yet!). While it's selected, grab the image and push it off to the side about halfway. Now you've got only half an image.
*NOW* paste the image back in. Leaving it selected, push it off the side the other way. Tweak it carefully (use the arrows), and get it so it just touches the remaining part of your image. Now you've got an ugly seam up the middle, *but the right and left will seam perfectly*.
Now tackle that ugly seam. Smudge it, rubber stamp it, make it go away and look like it belongs there, (this part is a pain, but this is the key to the technique. Practice at it.)
Now let's do it all again. Select 'all', copy it, push your image *up* this time, paste it in again and push it down. Once more we've got an ugly seam, this time horizontal. Make it go away.
Your *actual* seams at ther edge of the image will be perfect, the only potential ugly spots will occur where you welded the images together *in the middle*. Play around with different techniques to make this smoother, practice is necessary. Remember, the "rubber stamp" tool is your best friend on this one. -repeat as necessary-