One thing that I CONSTANTLY want to improve as a photographer is my editing skill. Not editing in the post-processing sense of the word, but in the curation sense, choosing which photos to display, which ones go well together as a series, what the most effective order is, etc.
I think we photographers generally overlook how important this is. Whenever we think of ways in which we can improve as photographers, improving the technical skills involved in making pictures seems to always be the priority. But since our photography is judged by what we ultimately decide to show, it's should be important to think about how we select what we show.
Two things that I do to improve my editing: 1) curate the work of others, and 2) asking others to curate my work.
The first one, curating the work of others, can be done quite easily, for example on Pinterest. Just start saving photos that you like. Then, revisit your collection of curated work after a while. You might find some common threads between pictures, and discovering these common threads may be the key to your improvement as you start incorporating these elements in a more deliberate way.
The second exercise that I found extremely helpful was having other creative friends go through my work and asking them to pick out their favourites. Even if they’re not photographers, they can still have an amazing eye. I did this when I was refreshing my portfolio (as I often do, since I sometimes get bored with my own portfolio after a while). I had a portfolio curation session with my mastermind group during which I projected a bunch of photographs on the wall, while everyone voted for or against them one by one. Just like editing a text you've been working on for too long, you can become too close to your own work and having other sets of eyes on it can be useful.
Some photos that I thought were amazing got 0 votes, and others that I had never shown before racked up everyone’s votes. After that exercise, the point isn’t to relinquish all control and just blindly choose the ones that accumulated the most votes, but to use this to inform your decision. Ask them why they voted one way or another. You may discover biases towards certain photos. As the artist, you still have the veto and how you use that power is an artistic choice.
If I’m presented with two photos, I can generally pick a favourite based on a gut feeling, and at times, that's all you've got. One might be better than another because of something intangible that you can't describe, and that's fine. But in other cases, I try to REALLY slow down and think about why I favour one work over another. It trains my brain, and hopefully I’m more aware about the things I want to avoid in the future.