Inner shadow as depressed button
How to use it
Apply an inner shadow to an element, offset downwards, assuming a top-down light source.
When to use it
If you want an element like a button to appear to be inset into the page because it was pressed in, an inner shadow is a good way to do it. This is because the top edge, which would normally be flush with the button, is now overhanging it, casting a shadow on the button itself.
You could also apply this style to elements are are toggled in or out, like a button that you press in and it stays in until you press it again. I don't see this style of button very often, though.
What to watch out for
The shadow doesn't need to be very strong to give the right effect. Try to keep it subtle, unless you have a reason not to.
If the element is not very obviously a button, it might be mistaken for something else like a field, since this technique is also used there to show that an area can be filled in. However, usually this style is only applied temporarily while a button is being pressed in, so it shouldn't be too much of an issue.