Tab and Space are common keyboard shortcuts if you want to move to the next field in a form and scroll down a page in a browser. But how about doing the opposite? That's when the Shift key comes to the rescue.
Yes, the Shift key acts as a reverse modifier for many of these shortcuts, with Shift + Tab taking you back a field, and Shift + Space scrolling up a page. Going through search results in a text editor works the same -- F3 goes to the next result, while Shift + F3 will go back to the last one.
This even works for undo, so Shift + Ctrl + Z will redo an undo... so to speak.
Keep in mind this isn't always the case. For example, Visual Studio (and many other text editors) use Ctrl + Y for redo, while Photoshop sticks with the Shift option. So best to consult the individual keyboard shortcuts for each program you use and consider this tip more of a rule of thumb.