One of my strongest shul-related childhood memories is of crossing a leg over my lap during the rabbi's speech, and my father telling me that this relaxed position was inappropriate for shul. [Update: My father has explained to me that he learned this lesson from the great Rabbi Leo Jung, z"l.]
This mental photograph has been very helpful to me over the years.
A dozen years in the rabbinate means that for a dozen years I entered and exited the shul [meaning the sanctuary, the actual room in which we daven] several times each day, and used that space for any number of non-davening purposes from rolling sifrei torah to preparing the room for davening to I don’t remember what. In the years before that I spent more than ten hours each day in beis medrash, as I do now in my new kollel life.
Bottom line: I have lived in “sacred spaces” more than I have lived anywhere else, and as a result I am perpetually on the verge of losing sensitivity to the kedushah [sanctity] of a shul and of a place dedicated to learning Torah.
One danger in this numbing is halachic: I’ll see nothing wrong with walking in there to shoot the breeze with someone, an activity which is prohibited. I’ve seen it happen; I’ve seen shul rabbis, holy individuals whose achievements dwarf my own, people I could not ever judge [although I suppose I do…], use shul sanctuary space for communal meetings, personal conversation and telling jokes.
In truth, some of those activities may be halachically justified by the fact that shul sanctuaries are often designed to be multi-purpose rooms. But the risk of desensitization is about more than the halachic Yea or Nay; there is also the greater effect on the way we experience shul activities – davening and learning Torah.
On this deeper level, desensitization to the shul space is part of a process of taking Gd out of davening and out of learning Torah. Davening and learning, without an awareness of Gd, become tasks, checklist activities, or even self-centered activities, rather than cornerstones of a profound relationship.
So I try to be careful to say Mah Tovu every time I walk into the shul; to relate to my children in a manner that also honors the space; to dress properly; to keep my speech limited to topics that are appropriate for the space.
And, of course, not to cross a leg over my lap.