Multitaskers do it Everywhere: Concurrently!
Philosophers think about doing it.
Photographers do it in dark rooms.

If a philospher does it in the forest and nobody's there to hear him, does he make a sound?

I'm professionally challenged at the moment and all I could think of is:

For housewives, doing it is a chore.

Yeah, I really do need a job.

the photographer ones not necessarliy true anymore...sooooo

Photographers make sure the positions right before doing it.

SAN engineers do it in a switch environment?

yikes that's pathetic. Ummmm...

Computer geeks do it until it megahertz. There, that's better.

We do it very graphicly, with lots of words and sound effects and in 48 pages or less...

Exactly! That always bugged me. Especially since it doesn't apply.....wait, is that TMI? :p That's why I love my sig line (Librarians have tighter buns!), because it's the closest thing to it.

How about "Librarians demand that you're on time"? "Librarians let everyone borrow some?" No, wait, I got it!

A librarian's services are free to all comers.

Actors do it center stage.
In the spotlight.
In front of a room full of strangers.
In two acts with an intermission.
8 times a week with Mondays off.

How about:

Librarian's do it in stacks

Librarian's due it every two weeks

Librarian's do it with HQ31 .C743 2002