I got a DSLR camera a while ago, and I took a photography course, but I really haven't learned much about how to take great photographs. I know about shutter speed, aperture, manual mode, ISO, and other vocabulary, but I don't know where to start for applying it. Any tips or lessons for someone who's not a complete beginner, but definitely not advanced?
I would consider myself as roughly fitting into that category. I have a couple main suggestions for you.
Shoot a lot. Doesn't really matter what format it's in, what camera or lens you use, whatever. Just shoot. After shooting, don't look at your photos right away, for the same reason that film directors often take a vacation between finishing a film shoot and beginning editing. Shortly after shooting, you have an emotional attachment to certain images, and that bias can cloud your judgement of them. Step back, wait a while, and then look at what you've got. If you allow yourself to become more objective to your own work, you will often be able to more clearly see the merits and flaws.
Shoot all sorts of different things. Sample many different types of photography. If one scares you more than others, especially try that one. You don't necessarily have to stick with one you don't like, but at least give it a fair, honest try. Even if you end up never doing that style of photography again, you may still learn something valuable from that initial experience.
Force yourself to be creative. One of my favorite ways of doing this is going out shooting with only a single, prime lens. Having a strict limit on focal length often means that getting a particular shot I envision is very difficult, sometimes even impossible. This forces me to stop and think, "Ok, what exactly am I going for here? What is important in this shot, and how can I get it with what I have?" In forcing myself to struggle for the right image, I frequently come up with even better ideas than my initial ones, and I feel like I've grown as a photographer for it.
Challenge yourself, and take photos. It is easy to say but hard to do.
An example of a challenge might be to use off camera flash successfully in a certain situation. Or maybe shoot a back-lit subject with a certain style. Get out and take photos, let others critique you, circle back and do it again.
You apply it by pushing yourself in difficult situations, with challenging lighting, or deadlines. You won't always be successful at first, but that is how you learn and that is how you grow. It all comes down to taking more images and learning from it.
Back in the film days, a common challenge was to shoot an entire roll of film on just one subject. Pick some sort of still life and shoot it from one angle, then from another, and another. Try to make each shot interesting and different. By shot 10 you'll be wondering how to take a different approach. By shot 20 you'll really be scratching your head about how you can possible get one more shot out of this. By shot 30 you'll just stare at the subject a little dumbfounded before finally coming up with one more idea. And at shot 36 (I always shot 36-exposure rolls of film) you'll have the epiphany about the perfect photo.
At least, that's the theory. :) Some shoots will be easier and some harder, some more successful and some less successful. It was a great way for me to focus on two aspects of photography: the art side and the technical side. Especially for the technical digital is a great advantage. With a common subject you can shoot and adjust aperture, ISO, and shutter speed to see how you can change the shot -- and get a better idea of how that stuff you learned actually applies when shooting.