Can the clock speed of my 2011 MBAir i7 be slowed to conserve power?
The Intel i5 and i7 processors have a number of features that will definitely conserve power or optimize it for those peak moments. You can't slow it down programmatically but you can be sure that the OS is doing all it can.

It's a good question, though, because an i7 processor running at 1.8Ghz doing nothing could be running at 1.2 and save power. In any case, I'm sure that one of the worst battery hungry things is the display. Make sure you dim it often when not using, use a lower brightness setting, turn off bluetooth if/when you don't use it, if you don't need network connectivity, turn wireless off, etc. Basic obvious ideas.

I have a free handy tool, similar to App Tamer mentioned by @ioi, that I occasionally use, Freeze Frame. You can simply stop certain processes from using any CPU at all, therefore letting the CPU to slow down. (Remember that with Turbo Boost it can go up!).

But I am not aware of a specific way to slow DOWN the processor. I'm sure Intel did its job there and the processor is clever enough to save as much as it can.