Tip #1
When using the clone brush, use a dabbing technique instead of dragging. It will usually result in a more natural look.
Tip #2
When you have complex items that you need to select, try using the "edge seeker" or "find edge" version of the selection tool. You can also make a rough selection of your subject and zoom in to remove the closer details with a small eraser brush.
Tip #3
Use "levels" instead of the brightness/contrast tool to enhance the highlights and shadows of your image without degrading the midtones. "Levels" will usually give you a better tonal range without destroying the detail in the image.
Tip #4
Name your layers by describing what that layer contains. If a new layer is for blurring part of an image name the layer "blur." This will help when you want to remove an effect without messing up any recent work.