You can also right click to pick up part of an image and then left click, but hold the click down and drag the clone.  Note where I picked up the section of the clock by looking at the brush outline.  When I drag the left click, see what it does?  It put down more of the image than I originally cloned.  So you have to be careful when you drag.  When I dragged the solid brown that I picked up, it only put down brown because there was no other part of the graphic to the right of it.

And again, here is the dragging with changes in Opacity.  I have used the dragging technique to fill in things such as desktops or cabinet tops where the wood was missing.

For seamless cloning, you need to clone carefully.  Look at the progressive graphic below.  I wanted to fill in the border exactly.  I first right clicked right above where I needed to fill in.  Then I carefully moved the brush down, staying on the exact same line.  Then I left clicked the clone in.  It filled it in perfectly.  If you waver off the direct line of where you picked up the clone, the clone will be off target.

When cloning, try to keep with the pattern, grain or line of what you are cloning.  When the clone is to repair missing parts of a graphic, you want it to look natural and exact as you can get it.  Clone near or right next to where you intend to drop the clone in.  As in the graphic above, I cloned right above the missing area.

Another interesting thing you can do with the Clone is to make a graphic more interesting or even much better.  Below is a beautiful bush in Ireland.  It's a shame there was so much black/dead space in the interior of the bush.  Well, clone it in!  I used a big brush size with a fairly high Opacity.  Be careful with Hardness though.  It was set for 50 in this case.  Too hard and you can see the Clone.  Now which bush looks better?