If you live in relatively cold climates, there are important steps to be taken before winter sets in to ensure that your sprinkler system is protected from freezing and the damage that can result from it. Below are a few steps to take in order to do this.
Step 1: Shut off the water
Turn off the main water supply to your irrigation system after the last use of the season. As a side-note, the main shutoff valve should have been freeze-proofed, i.e. installed in a heated room below the frost line and insulated to prevent freezing. It will do no good if your shutoff valve freezes and breaks.
For those with automated systems, the controller/timer needs to be turned off as well. You can set it to 'rain mode' which allows you to keep your set programs (valve run times, start times etc.) except that the valves won't open. If you don't have this option, you can simply turn off the controller by switching off the power supply. This will save you the electricity that would have been used up by the controller throughout the season. However, you'll need to reset your programs in the spring.
Step 2: Remove and drain valves
Take out your backflow preventer and pump water out of your risers before capping them. Drain water from the backflow preventer and store it safely until the winter ends. If you have valves installed at ground surface, remove, drain and store them away also. Alternatively, you can have pipe-heating cables installed on the backflow preventer and ground-level valves before covering them with insulation. This protects from freezing, but you'll have additional energy consumption to think about, not to mention the cracking that will follow an extended power outage.
As a precaution, have permanent insulation installed on your backflow preventer and valves; this protects them from any unexpected freezes before you're due to winterize the system. Backflow preventers are rather expensive, so you want to be sure they are completely protected. You can wrap them with self-adhesive foam insulating tape or fibreglass insulation, which is available in your local home-supply/hardware store. Ensure that you do not block any drain outlets and air-vents on the backflow preventers (consult your manual for instructions). Alternatively, many sprinkler stores have ready insulation blankets for backflow preventers and valves, which are much easier to install.
Step 3: Drain the pipes
Finally, you need to drain out sprinklers and pipes to ensure that water doesn't freeze therein and crack them. You can use your drain valves or blow the water out using air or suck it out using a shop vacuum (the last is very tedious since the vacuum will have to be emptied many times over). However, the blowing out is not something that you should attempt alone, no matter how handy you are with home-fixes. Hire a professional to carry out the blowing to avoid damaging your expensive system.