In a recent thread on Saka Gura on the Manhattan board, Jim Leff talks about how sake doesn't particularly like light, isn't meant to be stored very long, and doesn't last long once opened (in my personal experience, a week is the outer limits, although this depends on the individual brew).
Another interesting bit of trivia about sake, however, is that it doesn't like physical shock. If the bottle gets banged about before the sake reaches your glass, it affects the flavor quite significantly. Depending on your tastebuds, how it changes the flavor differs--my experience is that it gets watery-tasting.
Fun experiment to try at home (we did this in sake class; I doubt restaurant owners will want you banging their glasses about)-- pour a mouthful of sake into a glass, and drink, noting the flavor. Now pour the same amount into the glass, and slam it against the table, then drink. You should be able to tell why sake slammers should never become popular.
The change isn't permanent, so if you've given your bottle a shock, let it rest up for several hours and it should be OK.