Learning to shoot amazing photos on an SLR doesn't come overnight. Sometimes all it takes is a lot of experimentation, sometimes someone teaches you, and sometimes you get a great tip.
Have you scored once on shooting photos or a series of photos where the lighting and settings all came together and you got gorgeous results? You can replicate those results, and you don't have to have written anything down or memorized it.
Each of your digital photos will tell you what most of the used settings were — just click Apple + i (or right click and 'Get Info') on the file (that is, if you use iPhoto). Under More Info, the F-stop and shutter speed (on your camera) you used correspond to FNumber and Exposure Time, respectively.
To see how to get more settings in iPhoto, just read more.
Additionally, if you're using iPhoto, you can right click on the photo, click Show Photo Info, and you'll get the aforementioned settings plus the ISO.
Re-use those settings under similar circumstances (and you can adjust your settings from here using the old ones as a guide), and you're all set.
This is great for amateurs who are just getting used to their SLR, and even for more seasoned users who notice they shot some photos spectacularly once and want to do it again.