A great new feature in Lion is the ability to resize windows in a much more flexible way. Whereas Snow Leopard only allowed resizing a window using a “handle” in the bottom right corner of every window, in Lion placing your mouse near any edge of a window will result in a new “Resize” cursor indicating that you can resize the window from there.
If this wasn’t enough, there are a few other new resizing features that can make this even more useful.
Firstly, holding the Option key while resizing results in both sides of the window resizing equally. For example, if you have a full screen window and resize it by Option-dragging the left side in towards the centre, the right side will also move in by an equal amount. If you resize from a corner using this method, it essentially has the effect of anchoring the window by its centre rather than by the opposing corner as usual.
Secondly, holding the Shift key while resizing forces the window to keep the same proportions. This is similar to when you resize an image and want to avoid stretching it in one direction. If you double the width of a window, the height will be doubled as well.
Finally, you can use the Command key to resize windows in the background without moving them to the front. This has actually been around since before Lion, but it has become much more useful now you can resize windows from any edge.
Now, if you really want to be a pro, you can try using the modifier keys in combinations. You can also press and release Option and Shift at various points while you are already resizing to add and remove the constraints.
Bonus Tip: If you go to resize a window from an edge, but instead drag the mouse cursor in the wrong direction (i.e. drag up/down while resizing from the left/right edges) you can move the window around. This means you no longer have to only move windows using the title bar at the top.