What about the Michael Crichton "mosquito stuck in amber" idea? I've been curious about that for ages - assuming the amber is room temperature, would that even help preserve DNA for an extra day? I've been leaning towards "no", but I am extremely far from an organic chemist!
Encasing in amber really only shields you from outside things. Those include chemicals (especially oxygen, oxygen is _very_ reactive), life (bacteria) and possibly some shielding against radiation (eg light).
Those do encompass a _lot_ of the reason that anything decays, but unfortunately it's not _all_ of the reasons things decay. And over millions of years, the remaining decay mechanisms are enough that you're really not going to have much/any intact DNA. It makes for a good story though.
