I think a lot has been written on this. VTEC4-2 has a great post here:

www.s2ki.com/s2000/topic/211171-rear-view-mirror-installation/k

I think one comment that may help some users - is to understand how the mirror actually attached. VTEC4's post has clear pictures on how the spring and clip are assembled on the mirror. But another important part is to understand how the washer fits into the receiver (the silver part attached to the windscreen). The washer is intended to fit in the slot inside the receiver. See this picture (where I have disassembled the mirror so you can see just the washer).

LINK to picture of washer in receiver

Knowing that, you have to be careful when you are inserting the mirror into the receiver. I kept tightening the screw all the way down and then tried to insert it - but I couldn't get it to hold. You have to have the screw loose enough so that there is space between the washer and mirror - so that the washer can fit into the receiver. See this picture:

LINK to picture of gap between mirror and washer

It is a little awkward to hold - but you have to hold the mirror, with the gap behind the washer, against the receiver and then tighten the little screw on the mirror. You just have to hold it there until you can tighten it down. As you turn the screw, it will pull the mirror against the outside of the receiver while the washer is in the slot inside the receiver. The rails are the receiver are locked between the washer and the rest of the mirror.

This is probably obvious to everyone else but I had to fiddle with it for a long time before I could get - so I thought I would post some more pictures.

 

I am getting my windshield replaced tomorrow... Besides warning the windshield guy to "be careful to re-install the rear view mirror correctly", what other guidance can I give him?

I know these guys don't want to hear a long story and they hate people telling them how to do their jobs...so I want to be brief and to the point...but I also want to ensure that the mirror goes back on the car correctly...

Any suggestions, without telling me to send him links and photos and instructions?

Also, the work is getting done in a body shop by a sub-contractor because there's a bunch of other work being done to the car too...so I actually do not have full access to the windshield guy...

 

I dont think it would matter. I have also made sure the washer went into the slot correctly and tighten down. It still has a little play in it. From research, the aftermarket windshield anchor is slotted slightly different or the washer is deformed. I will probably put some 3m double sided tape in between mirror and anchor to stop the rattle. My windshield came from safelite.