Sometimes it seems like being a power user is the ability to pull off complex tasks as a unit. The truth is that much of the power comes from a small toolbox, liberally applied. Some of the most important tools are very simple: the modifier keys shift ⇧, command ⌘, and option ⌥.
One reason it's difficult to master modifier key tricks is because they're so abstract. Option can do one of ten things in ten different contexts: moving a word at a time through text, copying a file, etc. A common use of option in graphics applications is when resizing a selection or object. Grab and drag a handle while holding option and the action is mirrored, keeping the center of the object fixed.
So why would you ever expect to be able to apply this knowledge in, say, the Finder? The only thing you can resize there are windows. They certainly don't have grab handles, not even in the bottom right corner like they did in the classic Mac OS. But they don't need them; as of Lion, any part of a window border can be resized, edge or corner.
And, I discovered completely accidentally, holding option has exactly the expected effect, in the Finder or any application. It's a little weird at first, and can be impractical when dragging from a corner.
But I find myself using it quite frequently when dragging a side edge. If I'm single-tasking in a browser, I'll want the window front and center, but not full-width — it either stretches out the page design or just leaves blank space. Now I can quickly get the window just where I want it by zooming to fill the screen (another option trick, it turns out!) and then option-dragging from the edge of the screen towards the middle.
And, believe it or not, shift also works on edge and corner drags. That's most useful when trying to zoom media in apps like Preview or VLC, which don't automatically constrain window proportions. These are simple tricks, but when used at the right time they make accomplishing a task smoother, or quicker, or generally more powerful.