You can try to make it easier (probably a UX challenge) but most people don't take the time to be meticulous with setting up their filters. e.g. Gmail filters, Twitter lists, etc.
In the case of Twitter, people actually might if the interface was better. People did, before the interface changed.
Now, adding a large group of people to a Twitter list via web Twitter is such a painful exercise, I often turn to an API solution (tweetbe.at in particular) to do it.
Now, adding a large group of people to a Twitter list via web Twitter is such a painful exercise, I often turn to an API solution (tweetbe.at in particular) to do it.