Here are four basic points to keep in mind as you're looking to place a work of garden art. Longevity. Make sure that you and the art are compatible over the long haul before you go to the trouble and cost of placing it in your garden. (Will you always want that massive modernist sculpture or the 101 pink flamingos?)
Durability. Consider how long the art will last. If your heart's desire is made of wood, it will probably decay over time or require lots of maintenance. If it's made of metal, prepare to paint, or live with rust. Stone or concrete items will last longer, but they may crack from freezing and thawing or acquire a coating of moss over time.
Stability. Create a firm base that will resist sinking under the weight of the piece or shifting at the slightest breeze. Even if the object is at the back of a flower bed, somebody may accidentally knock it over if it's unsteady.
Use. Having in mind a specific idea, a favorite hobby, or a use for the artwork can help you narrow your choices. If you like birds, choose a birdbath that doubles as a work of art. To add the sound of water in a garden, choose a decorative fountain. Wind chimes add the element of sound and can also be eye-catching.