Ug, ug, ugh....
This is one of those "lies" about nihon-shu (what most foreigners refer to as sake
- sake just means "alcohol") that there's a proper way to drink it that's always proper...
First, take a look at Sake World
). John Gauntner is a pretty famous expert on it (and also a pretty nice guy), and he gives tastings and lectures all over the country. One of the reasons he's so famous is because he's a non-japanese sake expert and knows more than many Japanese sake specialists.
Now to answer the specific question, most nihon-shu should be served at room temperature or slightly chilled. Cheaper nihon-shu, like the stuff you get at the 7/11, should be served warm (or just done as a shot, really). Why do foreigners generally think it should be warm or hot? Well, until about 40 years ago, most nihon-shu was rougher, fuller, sweeter and woodier than it is now - and it did taste better warmed, because warming hid the nastier elements of the taste.
Nowadays, with major advances in brewing technology, good nihon-shu is generally designed to be served chilled or at room temp. The flavor comes out at those temperatures and it is quite delicious!
That's not a perfect answer - because there is some modern good nihon-shu that should be warmed (and that's what you should be drinking on a cold winter day). The best way to find out how to serve a certain kind is to ask! [/i]