Does anyone have any links/suggestions or guidance on how to manage things in the following scenario?

I've inherited a large (okay its probably only medium to small around the world but its big for me!) sharepoint site.

It has a LOT of subsites and each site has differing amounts of customisations for various purposes.

Not much (if anything) is stored outside of the sites (eg: form/source files/version control)

So was just wondering how this kinds of things are best stored for

a) further development purposes and/or back tracking

b) migrating or applying the same things onto another server (eg: if I build another server to mirror this one, so I can test things before they are live, how do you bundle changes, if at all? or do I just have to get really good at documenting changes while developing them so I can recreate it on another site?)