Ask HN: Simplest way to keep historical analytics

Can you recommend any "tricks" to keep long-term analytics of our web app up to date?

We monitor various vital statistics in our site.

Most of these are computed from the various tables that go back way in time, counting events (like user count over time, etc.).

It's really time we refactor all this as we grow and chop off old data from the app. However, we may lose easy access to some long-term stats (or access to new data we may be interested in the future), e.g. what if we wanted to look at signups by day of the week?

The Right Thing (TM) would be to refactor all the code and somehow provide accumulated data, plus the current counts, etc. Then bring back backups and query manually if we need new data.

Are there any ideas you found worked well to manage this?

(The app is in RoR)