I agree with Ken on that note, however, I have a few points of my own.
Having dealt in event planning for political campaigns and other organizations, I know how hard it can be to plan an event with many attendees, all on their own schedules, which is what we have when we do events for ZAB. However, the system we have now obviously isn't cutting it. The problem is this:
Let's say a month ago, when Wednesday in the West:KPHX was posted, I signed up to do SDL_TWR (nobody was, so this isn't targeting anybody). That was a month ago, and chances are that the person who signed up will have it slip their mind, new commitments will come up, etc. A good way to deal with this is a key of event planning, which is follow-ups! I think that the EC/CiC, whichever (and no, they're not exclusively the same person), should send out an email to each controller signed up for an event 3-5 days prior to the event, reminding them that they are signed up for the position, and requesting confirmation that they are still willing and able to attend. You'll weed out most no-shows this way, and open up spaces for other controllers who, judging to the proximity of the event, may now know that they will have the time to help out.
Just my two cents.