There are three major ways to grow as an artist; to strengthen your aesthetic senses and the quality of your work:
Emulate the work of artists you admire,
Experiment on your own, exploring the subject matter that interests you, and
Participate in critique
Emulation is very common, especially when getting started in art, and exploring subject matter of interest to you essentially describes doing art, so I’m sure you do that already. Critique, however, is where many artists are weakest.
Participating in critique means two things: evaluating the work of others and considering others' evaluations of your work.
For the most part, valuable critique will be given to you by people at or above your level of artistic accomplishment. The reason I say this is because people at different levels of accomplishment may express their ideas in different ways, and the closer someone is to your level of accomplishment, the better you’ll be able to understand where they’re coming from.
Participating in critique necessitates the use of some artistic vocabulary. Critique is only useful if it can be understood, so being able to express yourself in artistic terms is an important skill for growing your abilities. A great resource for expanding your artistic vocabulary is the very thorough ArtLex Art Dictionary. There you will learn that color, for example, can be described as “saturated, clear, cool, warm, deep, subdued, grayed, tawny, mat, glossy, monochrome, multicolored, particolored, variegated, or polychromed.” Knowing what these kinds of words mean will help you to be much more expressive in your critiques.
In photography specifically, there is no better site than PhotoSIG for participating in critique. PhotoSIG’s critique system is built in such a way that it encourages thoughtful feedback and rewards posts that the artist finds helpful. On top of that, the community is very large so nearly all work receives decent exposure, and there is a whole lot of talent to tap into. I highly recommend signing up for a free account and critiquing some photographs there.
You can read some critiques that I’ve written on PhotoSIG by visiting my photo critiques page
Flickr has a nice comment system built into it, but because it isn’t geared toward formal critique, it isn’t quite as effective at generating thoughtful, useful feedback. The best places on Flickr to solicit critique are the various “groups” created by certain photography groups and geared toward artistic growth in particular. Naturally, the Strobist.com Flickr group is a good place to start, though Strobist deals chiefly with off-camera flash images, so don’t try to get feedback on your landscape work there.
There are a few Flickr groups specifically for sharing critique, among them Photography Critique, The Photo Critique Forum, and Critique. I didn’t spend much time reading through them, but they each have hundreds of members, and hey, it’s free.