My method for this is first of all to create 5 categories - 'definitely see', 'probably see', 'maybe see', 'probably won't see' and 'won't see'.
Over the last week, I have made myself through the entire MySpace playlists for every artist on the first Big Day Out announcement. I figure this will give me a decent overview of their work. Based on this, they've been put into one of the above categories.
A couple of other factors also come into play - such as if I have seen them before, but I've managed to cull the list down quite substantially into a small number I'm excited to see. Given there are still two more announcements to go it's probably going to be pretty hard to actually fit everyone in when the schedule comes out, but at this point, the list for Big Day Out looks like below. The only act I won't miss for the world is Girl Talk - everything else fits around him.
Must See: Girl Talk, Dizzee Rascal, The Horrors, Kasabian, Peaches, The Decemberists
Probably See: Muse, Blue Juice, Gin Wigmore, KORA
Maybe See: Midnight Juggernauts, Tame Impala, Lily Allen, The Mars Volta, Calvin Harris, Minuit, PNC, The Checks
Probably Won't See: Powderfinger, Eskimo Joe, Magic Dirt, Karnivool, Kisschasy, Groove Armada, Ladyhawke, James Duncan, Dimmer
Won't See: The Temper Trap, Mastodon, Rise Against
Essentially the 'Must Sees' I will make absolutely every effort for. The 'Probably Sees' I will check out if they don't clash with a must see. The 'Maybes' are more likely to be things I stumble across or check out if I've got the time, the 'Probably Won'ts' will only be if a friend drags me along, and I will not see anything in the last category because I'm just not interested and I'd rather eat/line up for the toilet/go on a ride.
So that's my Big Day Out so far. The next couple of weeks I'm going to do the same thing for the (much longer) Rhythm and Vines line up, and then probably add to this list with the second BDO announcement. It's all VERY exciting.