Although it's too late for a lot of you that have already bought
compressors ...

You would have been better off buying just a small, portable air tank (5
or 10 gallons) and rig it up with the appropriate gate-valve and
connectors to attach to your sprinkler plumbing.

http://z.about.com/d/homerepair/1/0/..._tank_5gal.jpg

http://ep.yimg.com/ca/I/sjdiscounttools_2079_34448523

To fill the tank, either take it to a service station or buy a small
electric compressor.

I rigged up a 5 gallon tank so that I have a medium-sized electric
compressor connected to the tank as well as having the tank connected to
the sprinker system at the same time. This minimizes the time it takes
to disconnect / reconnect the tank to recharge it.

http://www.grainger.com/Grainger/items/1VAP3

What you need to purge a sprinker line is a massive blast of
high-pressure air - which some of you now realize comes mainly from
having a large tank that's been pre-charged. And only turn on 1 zone at
a time when you do it.

And you need a low-restriction gate or ball valve to let the air out of
the tank and into the line with as little restriction as possible.

If you connect a low-capacity / low-volume air compressor to your system
and find that you can't exceed a pressure of 30 psi then your sprinkler
system is probably self-draining and the air is bleeding out the drain.
You need more than 50 psi to force the heads up.