As with many of Photoshop's multitude of features, there are several ways to accomplish the same task. This Photoshop tutorial is designed to instruct on the proper use of the Clone Stamp Tool (S) which is used for duplicating pixels from their sampled area. Photoshop's rubber-stamp clone tool is a rather crude instrument which should be used carefully to avoid the stepped-effect or step-effect which is a dead giveaway.
To repair, rather than replace pixel information, the Healing Brush is a more elegant solution, mimicking the underlying color, luminosity, and tone of the underlying area.
the above sample photo shows the correct use of the clone tool to eliminate an unwanted object. By sampling the photo in an area that is as far away from the repair area (yellow circle) as possible, you will avoid an obvious "stepping" effect (see: center photo at top of page). Try to sample areas (white 'sampling' cursor) that have similar lighting, color, and detail.
With proper and limited use the cloning tool can produce acceptable results, although there are better ways to achieve the same end - see: Using the 'Selection Tool' to Clone Objects.
Photo Retouching with Photoshop's Clone Tool
CS or CS2 Users: If you are using CS or CS2, create a duplicate layer from the background by typing the key-commands 'Command>J,' or using the pull-down menu located in the Layers Pallet window. You can make your changes to this layer, then fine-tune the blending with the layer below using layer opacity.
CS3 Users: If you are using CS3, go to the "Sample" pull-down menu on the upper menu-bar and select "Current & Below" or "All Layers." This will make your changes on the top layer while sampling from the bottom layer.
Using Photoshop's 'Clone Source' Window
Open the "Clone Source Pallet" window by scrolling to Window>Clone Source from the top menu bar. By using the Clone Source pallet window, you can have predefined sampling areas that you can go back to over and over again, without having to resample.
Use a harder-edged brush to clone because the soft-edged brushes tend to create blurry and undefined areas where the brush edge overlaps to surrounding area.
Sample from an area that is as far from the working area as possible, carefully choosing similar background features, with similar tonal, and color charecteristics. I like to sample from multiple ares so that you are not repeating one specific sampled area in its totality. This makes the repaired area seem less obvious and duplicative.
