Depends on the board they use, but two 1/4" plugs to to a stereo mini (or whatever your input is). The problem with SB recording is that the vocals will be unusually loud in the mix compared to... well, everything else. If the sound guy is really friendly, he can give you your own aux channel, where you can do your own mix, but you gotta have good headphones that can isolate the band's sound. It helps to be there during soundcheck for this, so your recording isn't screwed up.
The other option is get a four-channel stereo mixer and a pair of mics. Two channels of your mixer will be the board feed, and two channels will be your mics, which are set up in the audience. The mics should balance out the vocal presence and add ambience to the recording. That's the best way, actually, but it takes the most work.