Some images in this post have been automatically down-sized, click on them to view the full sized versions:
Always do it manually. Always. You will get better results, and have complete control over the texture.
Heres my strategy, assuming I'm working with a 512x512 texture:
1: copy the tex.
2: double teh canvas width and paste another copy in the blank space next to the first. Flatten the layers.
3: On each side of the "line", identify key "features" that you would like to continue, and use the clone tool to continue them into the other side. This is where the skill comes in, and you will get better as you do it more.
4: Create a new vertical guide at 512 px
5: Using the guide, and being zoomed in at 100%, select and copy about half of the modified copy, starting at the guide and going right.
6: Paste the copied part.
7: Turn off the base layer so that all you see is the copied part.
8: Move the copied part so that it is aligned to the left side.
9: Turn on all layers, flatten image, resize canvas back to the origional size.
10: Repeat the above steps to make it tile in the up/down direction.
Also, heres a photo reference I took the other day that you can use for making tiles:
http://maze5.net