well.. just a very simple thing but comes in handy alot.. say you want to take an image of something and make it bigger.. but when its enlarged it becomes very pixelated a very quick way to do that is (these are instructoins for the gimp but can be done with PS too) go to filters>blur>gassian blur choose between 1.0 - 3.0 depends on how clear you want the image to stay, anyway thats it pretty simple and all that but handy
EDIT: check out the DT sig i made, its in my sig, i used this method on the water because when i made it larger and gave it the 3d effect it became very pixelated