I have got a very general suggestion. Make the events for those, who actually care about events.
I don't mean to complain about past events, but so far pretty much all of them (except preregistration) were organized like GM is announcing to the entire server that you gotta whisp him to get invited for some kind of event and people who are in middle of doing something, don't know if they want to come or not or they are not even 90 lvl whisp GMs and get invited.
Let's imagine that we organize some big PvP event again, like 40v40. It's about event on Frostwolf.
GMs announce that any volunteers to participate in the event must meet few conditions first. They gotta be lvl 90. They should have PvP gear. To balance the groups there will be certain amount of offensive classes and healers. Let's say 30-32 dps and 8-10 healers per raid. Obviously it will take much more time to organize a group (like 10 minutes instead of 2), but it's totally worth it.
I would love the idea of "you want to participate in the event? Come to GM". For example GM is sitting in Feralas and Horde players who want to join the group must go there, alliance must meet at Wetlands. This way is the best to separate randoms, who just whisp not even aware what the event is all about from people who read forums, wait for the events and really want to take a part in it.
If people organizing pugs can easily make attendants come to a certain place GMs, shouldn't have the littlest problem with doing the same.
It would ensure much better quality of events and bring satisfaction from it to those who care.