Pergola
A pergola or raised latticework supported by vertical posts is a good way to provide shade for your air conditioner while not reducing airflow around the compressor. Build your pergola out from your house and over the unit, leaving at least 5 feet of space between the top of your unit and the roof of the pergola. Train climbing plants to grow up the vertical posts and across the roof of the pergola to provide more shade. Keep your plants well pruned so they do not interfere with the compressor.
Awning
An awning on the side of your house can go a long way towards reducing energy costs in your home in a few ways. The awning will reduce the amount of sunlight beating down on the side of your house and increase the temperature indoors. Additionally, an awning placed over a central air conditioner compressor or window air conditioner will help keep the temperature differential optimal between the compressor and the outside temperature and help your unit operate more efficiently as well as increase its lifespan.
Trellis or Lattice
A trellis or lattice, erected some distance from your air conditioner or compressor, can also help shade your unit from the sun. Build your lattice at least 5 feet out from the compressor, and train a climbing plant, such as a grapevine or ivy, to grown up the trellis or lattice to provide shade for your unit. Constructing the lattice on only one side of your compressor, preferably the north-facing side, will help maximize airflow to the unit while minimizing sun exposure.
Precautions
Always check with an authorized repair technician for your air conditioner to learn about specific concerns regarding airflow and the effects of shading your unit. A technician can tell you where you can safely install a structure to provide your unit with shade and give you tips on keeping your compressor running smoothly for years.