I bought a bottle of Dom Perignon and have been told that it should be stored upright, not on its side. Is this advice correct? Also, how long can it be stored before I have to open it?

Generally, wines should be stored on their sides b/c otherwise, the cork gets dry, shrinks and lets air in. I have not heard anything different about champagne and have always stored my champagne on its side.

Champagne can also be stored for quite a long time, though aging may alter the flavor. I recently had a bottle of non-vintage veuve cliquot that my father had "aged" for four years. i LOVED the aged taste, but my mother hated it. The aging tends to make the champagne more yeasty, so i think it is mostly a matter of person taste.