do you have simple goal tracking set up?
if so, you'd be able to see for every visitors who fill out the form, (and got a thank you page) what terms and sources they used.
I find this to be the most valuable part of Google Analytics.
Plenty of other stuff too - but this is the big cahoona! If you don't have this, you haven't got it set up right.
Second down on the list is looking at page visit times or total page views on a per keyphrase basis. If you are paying a few bucks for a PPC phrase but it is only getting you 1 page view or just a few seconds, then either kill the word or improve the page. Alternately if the PPC phrase gets you good PV, but organic isn't, it raises the question "why" and may show an opportunity for improvement.
Can you see "segments" starting to emerge from the data? This was an eye opener for me.
Anyone else? I only do a few sites and get a total of 40 - 60 K visitors per month across those few - I suspect others with heavier traffic might have a few tips?