This tip is all about ways to categorize pictures in multiple ways, without wasting space with multiple copies.
One of my big challenges with photography is figuring out what pictures to use in different contexts. When I come back from a trip, I might want to pick one batch of pictures to print, and different batches to enter in camera club competition, turn into panoramas, and so on. Yet I'd also like to dump the whole batch of originals into one directory for ease of backups and viewing thumbnails.
The way I have settled on organizing these different categories is with OS X's "alias" feature. Drag and drop one or many files while holding Option and Command, and OS X will create tiny files that point back to the originals. (The icons have little curved arrows in the lower-left corner.) Photoshop can work (reasonably well) with aliases; you can view their thumbnails and open them for editing. What I typically do is "Select All" in my Originals folder, and Option-Command-Drag all the files to a different folder. If it's easy to select a group of files, like a sequence of pictures all of the same subject, I'll only select those files before dragging. The other folder will be something like "Panoramas", "Black and White", or camera club competition categories such as "Travel" or "Pictorial".
Once these folders are full of aliases, I can look at the thumbnails in Photoshop. At this point, the Trash Can becomes my friend. As I decide which photos are relevant for each folder, I can trash all the others through Photoshop or the Finder. The alias goes in the trash, not the original file. If the choices are not obvious, I keep a large set of aliases in the folder and flag only the ones I think are best. This way, the same picture can be flagged in one category, but not in another.
Aside from the freedom to trash pictures without losing the originals, using aliases has two main benefits:
You can use the same pictures in many different contexts without creating separate copies. I always wait until the pictures are converted to PSD before copying aliases. Each PSD is on the order of 20 MB. So it wouldn't be practical to file separate copies under Travel, Nature, etc.
Any edits you make to the picture, either through the original or one of its aliases, are reflected everywhere. For example, if you improve the contrast while preparing a photo for a camera club Nature competition, you'll see that same improvement when you go back to the Originals folder to make a slideshow. Any dust removal, Levels, and so on only needs to be done once. For pictures that I want to try as black and white, I'll create the Channel Mixer adjustment layer, but turn it off before saving, so the B&W effect does not show up in all the other folders.
I find that having folders of just the relevant photos is more convenient than assigning keywords in Photoshop for all the different categories and doing a keyword search -- less typing, fewer dialogs, and the finality of trashing photos that didn't make the cut is reassuring.
Now, Photoshop's support for aliases is not perfect. Although you can do things like flagging and ranking, where Photoshop remembers your choice, you can't go through aliases to assign metadata that's stored in the file itself. So any keyword assignment has to be done using the originals. I'll typically go through a folder of pictures from Yosemite and assign relevant keywords like Deer, Hawk, etc. and then filter by keyword to identify all the pictures to create aliases in my Wildlife folder.
The other big drawback with Photoshop's alias support is that you can't run a batch operation on aliases you've selected. If you want to boost the saturation of every picture by using Automate->Batch and an action, you've either got to go back to the originals, or open all the files and choose Opened Files in the batch dialog.
When sending photos to get printed, you typically need to crop them to a particular aspect ratio such as 4x6, 5x7, or 8x10. Although you can sift through aliases to pick out candidates for printing, be careful not to save the cropped version and overwrite the original. What I typically do is to make a selection using the Marquee tool with a fixed aspect ratio, and then do Select->Save Selection and give it a name like "4x6 Crop". I might even save more than one selection if I plan to make both a snapshot and a framed 8x10 of the same picture. I save the uncropped version after doing "Save Selection", so that I have the full-sized original, but at any time I can load it, load the selection, crop and save the cropped version under a different name for printing.