This entry focuses on traditional mediums and art which can be purchased.
One thing I'd like to stress is when you buy a work, you are investing in an artist's career. Art is not a loaf of bread, or a pair of pants; you're not going to eat it all up or wear it out (although the hunger for a new piece might come along).
Yes, not all art can be, or should be a commodity. However buying work at a fair price, especially from an emerging artist, increases the artists ability to further their career, which, in turn, will raise the value of the work you buy.
If you love an artist's work, if you see magic in their pieces, if you see a watershed moment just on the verge of inception. Buy. Their. Art.
Good art isn't cheap - cheap art isn't good, And if you procured good art for cheap then you probably took advantage of an artist.