Two things I think are most important-
1) If the mics are intended to get audience sound only, keep as much direct stage and PA sound out of the audience-recording mics as possible by way of their position, pattern and the direction they are pointing. The better isolated the audience reaction is from the stage and PA sound, the more usable it will be and the more control you will have over it in the mix.
2) Mic the section of the audience where the energy and musical excitement is best, rather than the section of bored looking people standing around and behind the soundboard in the back. Exclamations of joy beat distracted conversations every time.
He says this about PZMs-
"PZM microphones, hemispherical pickup pattern, placed on front-side walls work well in some situations. In some venues, the room geometry may be such that neither the band nor the P.A. is sonically "visible" to PZM mics on the front-side walls, so the audience response tracks should sound terrific."
I've found that 2 or 3 boundary-mounted miniature omnis, spaced across the front and taped to the front wall under the lip of the stage facing out into the audience can work really well. Alternately I've used an ORTF-like pair of cardioids attached to the stage-lip with a superclamp, with the mics hanging down underneath the lip, or a couple clamps with cardioids more widely spaced across the front and pointing straight out. In all those cases, the position of the microphones below the plane of the stage, with the stage itself forming a physical barrier really helps to block the sound from the stage so the audience sound is far cleaner. Consider that as an alternative to the more typical setup of mics on low stands above the stage facing out into the audience.
He says-
"Second, how do you want the audience to sound? The final mix should have a stereo spread that is exciting and realistic, yet contains some of the nuances of today's more contemporary styles of mixing. I like to spread the audience across the entire stereo spectrum, from hard left all the way over to hard right, with equal energy in between. To make this work, a minimum of three microphones are needed; I like to use four."
Wide omnis work well from farther back. The wide spacing helps in two ways- it decorrelates the diffuse audience sound and room reverberance for a big open and enveloping feel, and doesn't highlight only the people immediately adjacent to a single near-spaced pair. Outdoors, I add the cardioid facing backwards in the center between the omnis. It maximally rejects the PA and stage sound from in front, provides a full, wide and even stereo spread of audience sound in combination with the omnis, is easy to mount on the same stand as my main mics, and I have an extra channel for it when recording to the 6-channel DR-680. If the wide omnis were only for audience and room sound, I'd probably use sub-cards pointed backwards instead, since the omnis can sometimes get a bit too much direct PA sound if they are for audience or surround channels only. But I'm not using those wide omnis for that alone except for surround playback. For a 2-channel stereo mix the omnis are my primary Left/Right channels balanced with a more or less equal amount of forward facing center cardioid, which fills the center, provides good forward presence, clarity and direct sound like a SBD, and then mix in enough of the rear-facing cardioid for a good balance of audience sound. The isolation of the rear-facing cardioid having only audience and room sound in it is what makes it useful and allows easier level adjustment of that stuff in the mix.
This is gold-
"However, small clubs with low ceilings can present a particularly tough challenge. You must somehow get a picture of the audience in your stereo spectrum without hot-spotting the one jerk in the front who is muttering something about somebody's dog."