I have written in the past about the various aspects of becoming a better photographer, and my upcoming book is geared specifically to that task, but becoming a better photographer takes time and effort and study while quality improvement is about using the skills you have now to make sure that the prints you make (or the images you present on the web) are the absolutely best you can do, now.
So, I'm not talking about coming up with better compositions or doing a better job capturing peak action or more skill posing a model, I'm talking the technical aspects of the image which at most might require more care focussing or paying attention to depth of field or exposure, and many of the things simply require more care making the prints or processing the images.
With that in mind, here's a checklist of quality issues to watch for.
1) Have you made the print on a paper which doesn't detract from the image, neither overly textured or too glossy to even see the image without reflections interfering?
2) On careful but not nose on print inspection can you see the sharpening?
3) Is focus spot on?
4) Is depth of field adequate? It's not necessary to have infinite depth of field but the focus should be on the important part and adequate sharpness elsewhere not to be distracting.
5) Is image manipulation obvious? You should probably ask someone else since you already know what you did and probably can't unbiasedly judge whether over the top.
6) Are the tones in the image rich, deep, subtle and most importantly, as good as the images of photographers you admire? Were the highlights blocked in the original file or negative or slide? Was there ever enough detail in the shadows? Have you overused shadow rescue to the point of "straining" the image? Check Lenswork for black and white or Phot'art for colour. Frankly if your prints are as good as their mechanical reproduction, you're doing pretty well.
7) Is there too much clutter - things which don't add to the image? To be fair, this would require a reshoot or a rejection.
8) Is the overall level of brightness/darkness of the print too much? You may not need outside advice but you certainly need to walk away from the print and come back the next day before you decide.
9) Have you asked yourself what is the one thing you could do in editing the image (or dodging/burning if in the darkroom) to make this image better? Sometimes you change one thing, then you see half a dozen other things that could be improved - never leave a print knowing that it could be better if you did X to it. It's one thing to have doubts later, but to walk away already knowing what you'd do to improve it, well that just isn't ok.
10) Well, if you have fixed all of the above, you have done most of what can be achieved quickly. If none applied, then you were ready for some harder, more long term study and work.