Keyword demand measurement tips

I was wondering if anyone had suggestions on a good real time way (for weekly reporting) how we can measure overall demand for keywords, so that we can correlate demand and click throughs. Mainly so that when natural search traffic is down you can say the amount of people searching for x term was also down.

My first thought is low PPC bids for the terms but I'm not 100% sure if that will actually work.

Google trends doesn't show data on alot of our long tail phrases due to low(ish) volumes so it's not much use and it's also not that quickly updated more for broad trends in search behaviour.

I'm sure other people will have come across this sort of thing before and I wondered what you thought about it?