Maintain the proportions of an object while resizing it

To resize a selected object	Hold down
Proportionally from a corner	SHIFT and drag a corner sizing handle
Vertically, horizontally, or diagonally from the center outward	CTRL and drag a sizing handle
Proportionally from the center outward	CTRL+SHIFT and drag a corner sizing handle
While temporarily turning on or off the Snap to Grid feature	ALT and drag a sizing handle