There are ways to help spark your memory about where and why you shot certain photos. Use these tips to help you track your photos more efficiently.
Nearly all digital cameras automatically give the photos a file name that is a consecutive number as you shoot them. For example, the first photo you shoot might be 100-0001, the second might be 100-0002, and so on. As you download the photos to your computer, you can sort them by file name, and you then should have a list of the photos in the chronological order that you shot them.
Be sure to set the correct date and time on your camera before you use it. As you shoot your photos, most digital cameras will stamp each photo with the date and time that you've set with the camera. (You may not see the time and date displayed with the photo, but the time and date are stored with the photo's data.) You might spark your memory by seeing the time and date the photo was created.
If you're shooting a series of photos of a large group of people or an odd scene, you might want to shoot a familiar landmark or a sign immediately after shooting the group or the scene, something that you're sure will help spark your memory later. Because the photos should be in chronological order on your computer, the landmark or sign photo should immediately follow the photo(s) they're describing.
For those learning to use a DSLR camera or trying out various manual-control settings, you may end up shooting several photos of the same scene, trying out different shutter speeds or aperture settings. Later, though, you might have a hard time remembering exactly which settings you used on which photos. To fix that problem, shoot a photo with certain settings. Then, write those settings in a notebook and shoot the next photo of what you've written. Later, when you have the photos on your computer in chronological order, you can see which settings may have worked best under those shooting conditions, hopefully helping you improve your photography skills. (This data also is stored in the photo's EXIF data, which is included with the photo file, and you can view it there.)
Along that same line of thinking, carry a notebook and a sharpie marker with you. As you shoot a set of photos, write down some keywords that will help you remember the scene or people, and then shoot a photo of the notebook after shooting the photos of the scene. This works especially well for large group photos where the faces of the subjects will be very small in the final image. If you're only interested in one or two subjects in the group photo, be sure to write down in your notebook the location of the subject in the group photo -- such as "third from the left, second row" -- and you'll then be able to find each subject later in the group photo, as the photo of the notebook with the writing should be directly behind the group photo in question.