I just discovered this show a few days ago (actually got interested when it was mentioned during an article about another show that I watch). I think that it's wonderful right down to it's Lemony Snicket narration. I got hooked only to hear rumors that it was in ratings trouble.
As a veteran viewer of great shows that have been canceled (even with good ratings numbers) I have come to believe that creativity is always in danger on TV. I don't know who they give the Nielsen set power to (since I have never met anyone who knew anyone who had a ratings box on their TV) but I have my doubts as to whether I'd really want to know them considering the ratings polls.
However, my point is digressing. The real message that I am trying to get out is that the viewer does actually have more power to save a good show then they realize. More, in fact, than a Nielsen rater. Just as the time to save a patient is before they die (Ned possibly excepted) so you have to save your show before it is on the cutting block. After network bosses make up their mind it is almost impossible to change it. It is a fact though, that one fan letter counts far more than one Nielsen rating. Email is great but does not require the effort of pen,paper and mailing. For every fan who actually takes the time and trouble to let them know that the show is appreciated the networks know that there are thousands who must feel the same way (actually there is an actual number that they have determined, according to a screen writer that I once heard, but I forgot what it was). The other thing about ratings is that they serve a purpose. They are a tool that TV uses to convince advertisers, in an intensely competitive market, to give money for time slots. The only other physical tool that they can push on advertisers is fan letters. In your letter you may want to state your age group for their records and point out why that group is a good one to support.
Side note: On the campaign to save Farscape we wrote to the advertisers, thanking them for their support. The result was mixed. HP (Hewlett Packard) said that it made no difference because they don't care what is showing, they just pick by time slot and spread it out on the channels. Kentucky Fried Chicken on the other hand, was impressed enough to state that they would sponsor the show on any channel that cared to pick it up. Entertainment Tonight picked up on the campaign and gave it publicity. Although it didn't save it (we didn't start at first rumor and so it was too late) it did at least lead to a Mini-series ending.