Today's lesson is a very simple one:
Save all your images.
Even the really bad ones.
I can not tell you how many times I have gone back through year-old files and found usable images that at the time I thought were only worthy of the trash can.
As my skills progress, I find I'm better able to edit and manipulate images.
Sometimes I can save the entire image, other times I use a portion of it or as an overlay, and in many cases those old images serve as great creative-springboards!
ETA: A lot of you have mentioned that you just don't have the space to save all those image files. And I totally get that! My computer's hard drive was at max capacity about a year ago. So I took the plunge and bought the Western Digital WD Elements 1 TB External Hard Drive. That's a terabyte! 1,024 gigabytes. It cost me $70 and I am so glad I got it! Now I can shoot in RAW + jpg mode and not worry about the file sizes.