If you create a texture or if you have already a picture that you want to use as a texture, you can make it easy tileable with Photoshop.

So let's begin with finding a nice picture that can be used as a texture. I've choosed "old stone wall little cement" from Mayang.com , a very nice site by the way. It's a great texture but with a small problem: it's not tileable. You can test it by yourself in Photoshop.Take a close look to the corners and borders where texture is slices.

Now open the image in Photoshop and bring it to the desirable size, I'll make a width by 900px. Hit Ctr+ R to have you rulers in PS, and drag from rulers space a vertical guide to the exact half of weight ,

 and

 horisontal guide to the exact half of the vertical size (in my case is 450px of width for V ruler and 337px of height for H ruler). On the layers panel hold Alt and double click on the name of the layer to unlock it (if it is locked, obviously).

Cut the right side of the image and paste it on a layer above.

 

Change the left side with the right so, that the texture fills the entire canvas.

Now merge this layers by clicking Ctr+E. 

 

Observe the margin that did not

 

 fits well. So this is the most important part of the work: use clone tool to make the border invisible and to complete the stones and other parts of texture that did not look ok.When using the clone tool set the opacity of the brush by 75%, that will allow you to make easy edges for the stones by different colors. Try to copy the parts that are looking closly to the area that you are working on. Now, after you've finished the cloning procedure you have the texture that is repetitive on the x axes. procede with the same technique for vertical tile: Cut on horizontal direction the bottom side, paste it on a layer above, change bottom with the top size and merge the layers. With clone tool make the transition on vertical smooth and nice, also try to eliminate the large dark side of shadow from external objects.  

When choosing your image for texture make shure that it has the same gamma of color and it will be better if it will have a multiple elements (in my case a lot of stones). Test tileability as you did in the start of this tutorial. That's it, hope you like it!

The final texture you can find here