On the other hand, the best encores I've ever seen were these:
1. Wilco - They basically came back, played twenty more minutes, then left the stage. To thunderous applause, they returned a second time, playing an additional twenty-five minutes. This was after an already hour-plus set. The crowd wanted more and Wilco delivered to a point of utter satisfaction. Though I would have loved for them to basically never stop playing, I was extraordinarily pleased by what they gave.
2. The Sadies - The Sadies hadn't planned an encore, I'm ninety percent sure of it. You don't play a song like "Tiger, Tiger" if you're planning on coming back. The audience response moved them back onto the stage, at which point Dallas Good informed us that we'd been a real good crowd and that they could probably play three more. One audience member yelled, "Ten more!" At his behest, Dallas bumped the number up to six more.
3. The Constantines - Played two songs, one of which was an absolutely incendiary cover of "Thunderstruck." More importantly, they did it after maybe thirty, forty-five seconds of cheering.
4. The Arcade Fire - Finished with "Rebellion (Lies)" and then left the stage. The audience was still way too busy singing. They had to actually wait for the melody to start dying down a little before they came out and played a couple of songs for an encore. The real kicker, though, was that they ended with "In The Backseat" and then brought it out into the crowd. Nothing like wandering through a group of people pouring out their heart to an endless repetition of the final few bars of one of your songs, I'd imagine.
Really, there are two or three things that should happen with an encore. First, let the crowd dictate whether or not there's an encore. Be prepared for one but if it's more a matter of obligation than a matter of the crowd wanting it, just let them go - and trust me, you'll know within seconds whether or not a crowd wants an encore so don't be a dick and just get out there. Second, make sure the songs you pick are absolutely worth people staying. Third, if the crowd really wants more, don't just play one or two songs and then go home. Jesus, they're loving everything you do, it'll probably be a total joy to play. If it's a chore for you to do encores or play more than two songs when an entire crowd is eating out of your palm then you've possibly chosen the wrong career path.