As soon as you notice a jam beginning to form, stop the shredder to keep things from getting any worse. This gives you a chance to slow down, assess the situation and prepare to fix the jam. Jamming signs to be watchful for include a slowing down of the paper moving through the shredder, an outright stop, and a whirring, "straining" noise that should be readily obvious. One reason that shredders sometimes jam is that there isn't anywhere for the paper to go once it is shredded because the wastebasket is full. If your wastebasket is full, try emptying it and re-trying which may be enough to resolve your jam. If the jam still won't clear, proceed to the following steps. Because jamming is a common problem, most modern shredders come with a built-in option to run in reverse. Switch the shredder to its "reverse" option (usually there is a well-marked button on the top of the shredder) before plugging it back in. Ensure your fingers or any other tools are not near the shredder opening when you plug it in. Reversing the shredder will usually clear a minor jam in a matter of seconds. However, in especially bad cases, the shredder may jam again when it runs in reverse. In this case, unplug the shredder again, switch it to "auto" or "forward" (the precise option may vary on your shredder) and plug it back in. Be prepared to alternate between auto and reverse as needed. A jam that's bad enough to gum up your shredder when it runs in reverse may be bad enough to jam it yet again when it runs forward for the second time. However, by switching between forward and reverse, it's almost always possible to gradually work your paper out of a jammed shredder. One of the most common reasons for a shredder jam is simply that too much paper was fed into the shredder at once. Once you clear your jam, try shredding a smaller amount of paper if this was the cause for the jam, the smaller second load should pass through with less difficulty. If you still can't seem to get your jam to clear after alternating between reverse and auto/forward, you may have an extra-severe jam that needs to be cleared manually. Never fear, consult the section below for further help.

Summary:
Unplug the shredder. If needed, empty the wastebasket. Switch the shredder to "reverse" and plug it back in. If the shredder jams in reverse, switch back to auto/forward. Reduce the thickness of your load before re-shredding.