I wasted many good sheets of photo quality paper finding out that printing directly from Photoshop was creating a much darker print than what you actually viewed on the monitor while within Photoshop. The same holds true if creating cd rom or VHS videos from Premiere. So simple - yet so needed. Same keystroke for it to return. Finally see the effects of changes without distraction.
All design needs the following.
I'll use typography as an example. If you use an ugly type face on the front cover of your piece, pick-up that same face in a smaller version inside or perhaps as an occasional initial cap. There should always be one item on the page that is the primary focus - don't let two items fight for control. For instance, an image of people is sometimes better served by using the green channel instead of just converting to greyscale. If your small web text is a little faint because of the anti-aliasing, try doubling the text layer.