Breaking my rule of 'always avoid alliteration', how about:

-Passionately preserving the past
-Passionately preserving prehistory
-Our passion is preserving prehistoric places
-Preserving prehistoric places for people's pleasure

OK, OK, maybe the last one's a bit silly....

So, allow me this indulgence... a bit of Shakespeare?
'Pity, you ancient stones, those tender babes' -King Richard III: IV, i 'Like stones of worth they thinly placed are, ' -Sonnets: LII

The thing is, Heritage Conservation is such a vast field I think we need to define a role for ourselves, rather than merge into the general mass. After all, the National Trust we ain't, and most people here although they "care" seem more drawn to "fight" than the average heritage enthusiast. That could be our role, publicizing and running specific campaigns. It would keep the thing fresh and enthusiastic.

("Publicizing" again. Sorry Mr Windz).