Building your collection
When buying art, choose what really appeals to you rather than what you think might be a good investment. There is no guarantee that the value of a piece of art will increase over time, but if you buy what you love then this should not be a concern.
Some people have a preference for certain genres, like landscapes or abstracts. Others may prefer the look of certain media, like a pastel or watercolor painting. A particular individual may be buying to fit a theme, like a Japanese themed home or room in their home.
Remember that an artist retains copyrights on works sold unless specific arrangements were made for sale or lease of the copyright.
Caring for your collection
Many factors can damage artwork over time. Moisture (water) and sunlight are probably the biggest problems for paintings and drawings, which is primarily my focus. However, chemicals from the materials in contact with the artwork (like acids in some papers), and even insects can cause destruction. Avoid displaying or storing art in direct sunlight, extreme heat, or high humidity, or where these factors can fluctuate frequently and to an extreme. Just think about what museums are like. This doesn't mean you have to have your house controlled exactly like a museum environment, though.
Some types of art may be durable enough to hold up fairly well in these potentially destructive conditions (like stained glass or sculptures).