Plundering your own perspective for material is also a way of spending the principal. Your current perspective, especially if you are young, is pretty limited, and the world it encompasses is constantly shifting and evolving. Maybe if you’ve worked in a field or government position for decades, your perspective is substantial enough on that particular issue to sustain a lifetime’s worth of blogging. But for most of us that’s not the case, and our perspective very quickly becomes a shtick, a rote performance, a reflexive mechanism for avoiding critical thought that rivals the "ritualized viewlessness" of traditional journalism. Pretty soon, you’re saying the same thing over and over, and realizing how often you use the same arguments, back them up with the same old links, etc. I realize this problem is not the internet’s fault, as many print-newspaper opinion columnists are some of the worst incarnations of it, but I think the pace and exposure of blogging intensify it.
To be a fresh and relevant writer means, I think, that you have to be something like a fresh and relevant person, one who reads slowly and widely, has idiosyncratic interests, goes new places, meets new people, and regularly changes their mind. Feeling my own perspective plundered and empty over the years has pushed me to appreciate the value of, if we use Nolan’s terms, "building up the principal."
The more you can be forced past your current perspective, and not just by other bloggers and journalists, the better. The more you can participate in something besides consuming media and blogging, the better. The more you can really learn about something the better; good writing can’t survive all that long on nothing but voice and other people’s reporting.