You've worked on a project that's generated multiple versions of many files with next-to-useless names, and now you need to send the best ones to your colleagues. How can you efficiently rummage through the candidates, when even the apps that created them can't give you a good overview of multiple files? All you need is the often-overlooked features of OS X's Quick Look.
Start with selection techniques
To use multiple files in Quick Look, you must first select them in the Finder. What's the best way? Click one filename and then Shift-click another to select the two items and everything in-between; for noncontiguous files, switch to Command-clicking. Drag to select some items in a window, or get them all with Edit -> Select All (Command-A). To select files that are in different folders--as long as those folders are in the same parent folder--set the enclosing folder's window to List view, expand the subfolders by clicking their expansion arrows, and Command-click to select the target items.
After you've selected multiple items, open the Quick Look window by selecting File -> Quick Look [number] Items (where [number] is the number of items you've selected). Or, press Command-Y or the spacebar.
Since Quick Look's floating window remains open while you work in Finder windows, it's really important to keep track of which window has "focus"--in other words, which window reacts to certain keypresses. You might, for instance, press the right-arrow key to select a different thumbnail in the Quick Look window, and instead accidentally change the selection in the Finder window if the Finder window has the focus.
There are visual clues that can help you keep track of where you are, but they're frustratingly subtle. The Quick Look window has a bigger shadow when it has the focus. (Yes, that's all.) When the Finder window has focus, selected file names are highlighted in color; otherwise, they're highlighted in gray. The Finder window's title bar doesn't dim even when the Quick Look window has the focus, because the Finder window is still the active window.
If you use commands like Command-W for Close, or Command-F for Find, or anything else that the Quick Look window can't handle, your keystrokes pass through to the Finder window, even if the Quick Look window has the focus.
Move through previews
Once you've selected multiple files, opening the Quick Look window displays the first item. The right-arrow or left-arrow key displays the next or previous file, respectively, cycling around to the beginning again if you keep going. You can also use a two-finger swipe on your trackpad to move around. (But press the up arrow or down arrow, and the key press takes you to the Finder window, deselecting your multiple-file selection in favor of a single item.)
Steady the size
As you flip from one file to the next, the Quick Look window changes dimensions to accommodate the size and shape of each. If you find this as distracting as I do, resize the Quick Look window a bit. Once you do, it stays there, and changes the size of the contents to fit.
Get an overview with the index sheet
The index sheet is the crux of Quick Look's handling of multiple files, providing thumbnails of all the selected items. View it by clicking the Index Sheet button (the grid-like button) to the right of the Next and Previous buttons in the top-left corner of the Quick Look window. Click on a thumbnail to move back to the single-item view. Press Command-Return to toggle between the two views. In the index sheet, select an item by using the arrow keys (the item is surrounded by a blue halo) and display it alone by pressing Return.
Enlarge or shrink the index sheet's thumbnails by changing the size of the window; rather than changing the number of thumbnails displayed, this changes their size.
See the big picture
Sometimes the Quick Look view isn't quite enough. You might want larger versions of either the thumbnails or individual files. If you're reviewing images, you might prefer a black background instead of Quick Look's white. How about an automated slideshow to help you review the files? That's all available in full-screen Quick Look.
Switch to full-screen mode by clicking on the arrows in the upper right corner of the Quick Look window. They show only when an individual file is displayed, but once you're in full-screen mode, the index sheet is accessible. You can use the same keyboard commands as in the Quick Look window: Command-Return toggles between the index sheet and a single item, Left- and Right-arrow keys or two-finger swipes control the parade of individual items, arrow keys select an item in the index sheet, and Return shows the selected item by itself.
Keep things true to size
To display items "full screen," images are enlarged (keeping the original proportions) to fill the screen; this also happens in a large Quick Look window in the Finder. To avoid the distortion that results in either case, press Option to make the image shrink to its true size. You can keep the Option key pressed as you swipe or use the arrow keys to move from one image to another to see each one at its original size.
Take advantage of on-screen controls
You'll see a floating toolbar when you view an individual file in Quick Look's full-screen mode; it appears as soon as you move the arrow cursor, and fades out of the way after a few seconds. Press the Play button to start a slideshow so that you can leisurely peruse the large-scale version of each item. Click the Index Sheet button to go to that view, or the Add To iPhoto button to add compatible files to your library. Use the Exit Full Screen button (or the Esc key) to leave full-screen mode. The Close button not only exits full-screen mode but also closes the Quick Look window.