Fishing expeditions go something like this: We look through /pol/ for threads likely to contain old /pol/lacks, and look for any posters lamenting the decline of /pol/. A poster responds to them and says something along the lines of "remember the merged board where the horse fuckers scared the redditors off?" If that gets a positive response, another poster (or the same poster) responds with something like "did you know they made their own website?" Or for less subtly and more risk, just send them a link.
Fishing expeditions using these approximate tactics have proved effective and gained us several new members. It's probably also worth thinking about how to fish from /mlp/, pone, or other sources of possible new members.