Do you know the difference between f2.8 and f4? How about f9 and f11? I mean, what your lens does to your images, at these and other apertures, in your typical shooting conditions?

If you don't know, I really recommend that you find out. Shooting film is a brilliant way to establish this for yourself because you haven't got the luxury of immediate capture and the temptation to delete and forget. Take your time and set up your camera on a tripod and go through the apertures. On another roll, shoot your typical shots but vary your apertures. I have shot rolls just 'wide open', at my fastest aperture, just to see. Study the results. You will then build up a mental picture of images and variations in aperture that will filter into your shots. Your aperture choice will become more informed and, I hope, your pictures better as a result. Don't worry if you don't shoot film, this works with digital too, but you need the discipline to see it through.