I use the following on three separate speaker pairs in my backyard:
http://www.outdoorspeakerdepot.com/outd ... trols.html
What is nice about them is that they come in a nice weather proof conduit box, and have built-in impedance matching. This means your amp sees the same impedance regardless of the volume adjustment on the different pairs. The other nice thing about them is that you can mount them near the speakers, not the amp. So, you can be sitting down at the patio table and have a nearby volume control to turn things down without affecting other zones. This is effectively similar to a zone control on a receiver, but gives you much more local control. Switch boxes are okay, but then you have to run back to the receiver to turn on or off a particular zone.