I tend to give them a call, and attach a cover letter to the agent stating with a smiley face or something friendly that I wished it was a full price offer and that we all wished they were all full price or better, and then go into some of the compilation of market activity in the area of the home... making certain that I say something like, I know you have gone over the recent market activity with your client (doubt that they have, but maybe... just in case...here it is...), but looking at the comparables, etc., etc. and that I wouldn't be doing my client justice if I didn't re-point these out. yada yada yada...
I attach the comps in a little package attached to the contract...
If it is really low something about market price being what a willing and able buyer will pay... and attach a preapproval (hopefully) letter at the amount of the offer.
I have had good results with the former...and what one would expect from the latter

. I work hard to meet as many other agents in town, by being active in general realtorĀ® activities, and have worked hard,
following your soulful practices to establish myself as "One of the good guys to work with" among other brokers and agents. It sure helps when you can present a lowball to an agent and smile, like it is the best thing since sliced bread....
YOU know it's a lowball...SHE knows its a lowball...
you keep smiling,
she keeps smiling with a big sigh and , "Well, I have to present it." I respond with a smile and say," Yes, you do. Let me know if there is anything else you need from me." Normally there is either a good natured response of, "No, you have done quite enough." or "Yes, about twenty thousand more dollars on this offer."
I too am interested in other responses.... and OH, Merry Christmas everybody!