It all comes down to viewership. If the shows don't get enough views, no matter how good they are, they are going to get canceled. The studios, at times, don't help things as they don't market the show or give it a terrible time slot, or put it up against established storng-holds of the other networks. "Firefly", is one that comes to thought where the studio not only gave it poor support and lower viewship time slot, but they also aired the episodes out of order, thus creating some confusion among the viewers.
One good thing these days is that if the production costs aren't ridiculous, some of the smaller networks can pick up canceled shows and bring a second life in to them. Also, if the smaller networks have original content and its good, they don't need as many viewers to call a show a success. At least shows have a better chance of making it to the next season.
I do like the strategy of waiting for shows to hit DVD, but i reality, I'm always looking for the next great show and rarely could wait. My latest test show is "Orphan Black" and so far, so good. :)