Turn on the water for your sprinkler system and allow the water to run for about 5 minutes to ensure that it has reached through the whole line of sprinklers. While the water runs, look for areas that the water is not spraying correctly as signs of debris.
Turn off the water supply and watch to see which sprinkler head is the first to turn off and the last to keep spraying water. These are the high and low points of your system.
Remove the sprinkler head from the low point sprinkler first, and activate the water again. The water will run to this area last, and will blast out any dirt and debris. Allow the water to run until the liquid from this sprinkler head runs clear, and then turn off the water.
Remove the next sprinkler head and repeat the process, until you have worked your way to the high point on the line. Allow the water to run for 1 to 2 minutes with all heads removed to ensure all debris is out of the system.
Deactivate the water and reattach all sprinkler heads. Repeat as necessary for other lines in your system until you have flushed the entire system clean.