1. Find a texture that you want to repeat.
For this, I used a texture from Blualien on Flickr. I liked it because it’s very interesting and making it repeat seamlessly looks a little difficult. That shoud make for a good example.

2. Copy the image and place it directly on the side of your first image.
After you copy it, put a guide line between the two images and then merge the two layers. Your guide line is there because you are going to split the two layers up again in a second.

3. Clone pieces of the image so you can’t see the seam line.

First hide your guides (ctrl+h). Then select your image (ctrl+click the layer *or select with your marquee tool (m)). Once your images is selected, use the clone tool (s) to hide the seam line. I usually use the clone tool with the brush size at about 45 and the opacity at 80%. With your clone tool, select an area to the left (alt+click) and brush over the line. It should end up looking like this:

4. Split up your new image.

Now that you have your image looking like one solid piece, turn you guides back on (ctrl+h) and cut the image in half again. (select one half and press ctrl+shift+j). Next, move the second image back on top of the original image. Then in the top layer select the right half of the image and delete it and merge the two layers. It should look like one solid piece again. By this point, you can duplicate the image to the left and right and it will repeat smoothly.

5. Repeat all steps vertically

Create a guideline below the image, duplicate the image, and place the top layer below your original image.

6. Clone over the seam line

Merge the two layers first and select the whole image. Then use the clone tool and clone pieces of the image over the seamline until it is seamless like below:

7. Finalize the tile

Just like with the horizontal repetition, cut the image in half by turning your guidelines back on, select the bottom half of the image and hit ctrl+shift+j. Put your new image above the original and delete the bottom half of the top layer. It shuold look like one solid piece.

7. Duplicate the image to the left, right, top and bottom.

Now, you can export the image out as a background and have it repeat along the x and y axis like below. With any complicated texture, you will still know that it is repeated but nobody will know exactly where the seam is where it’s repeated. This will reduce your file size and still let you repeat a cool texture.