Lager beers should be fermented and aged at low temperatures. Depending on the yeast, the beer should be fermented at about 45-55°F. This requires a spare refrigerator and temperature controller. That is something I do not currently have. As a poor but necessary substitute I take advantage of the cold Minnesota winters to lager. Placing the carboy in a closet on a cement basement floor gets the fermentation down to 56-60°F. That's not perfect, but it works, sort of. I have made a number of lagers this way and have always been content with the product.
The fermentation is not as "clean", but it is still fine beer. I drink it. It is not as crisp as a real lager, but I drink it. It tends to be hazier, but I drink it.
Another important step in making a lager correctly is the "lagering." Lagering is done after fermentation is complete. Usually the beer is transfered to a secondary and cooled to 34-40°F. This is stored at this temp for up to 2 weeks. During this time, the beer "cleans up." Chemically the goal is to allow the levels of diacetyl, acetaldehyde and sulfur compounds to decrease.
One trick homebrewers can do is allow the fermented beer to warm to room temperature for a couple days before lagering. This is called a diacetyl rest and allows the yeast to convert acetolactic acid into valine instead of converting it into diacetyl (a.k.a. butadione). This helps clean up the beer.
Yesterday I brewed a dark lager. Something along the lines of a Beck's Dark. It is fermenting at 60°F right now in my basement. We are at the start of another cold streak, so that temp should drop further as the current cold arctic blast removes kinetic energy from my house.
 this summer will be spent watching the classifieds for a free refrigerator :) I hope to have lagering capabilities by the end of summer.
 just like the good old days when lagering was invented.
 what a "clean" fermentation is could be debated.