What kind of microphone setup is ideal for this situation?

Elevated uni-directional boom mic on a stationary pole and extender?

My church uses almost invisible microphones that the pastors and band members have in their ears. I have not worn one myself, but if it works for a church sermon, it’ll work for a lecture.

@Fred931 this is a good idea, however we have several filmers using the same two studios, and this wouldn’t be very sanitary.

I think this is what they look like. Notice how they don’t actually don’t go in-ear.

As most TV studios have successfully used clip on mikes for decades I would suggest that you are doing something wrong. A good setup has AGC/AVC and that should eliminate volume changes provided you have sufficient initial gain. How are you transmitting from the clip-on?

You could go for boundary microphones:

http://www.google.com/products?client=safari&rls=en&q=boundary+mics&oe=UTF-8&um=1&ie=UTF-8&ei=bLb4SsnyDYPitQPWtuidAQ&sa=X&oi=product_result_group&ct=title&resnum=1&ved=0CB8QrQQwAA

They tend to pick up sound from a narrow angle and are generally very open sounding (they don’t suffer from proximity effect in most cases). One on the podium, and one in the same general vicinity off-axis; that way you have even coverage. The only rub is, if you have multiple mics, you’ll need a mixer and someone to monitor where the speaker is, otherwise you’ll run all the mics at the same level (which may increase the noise floor).