From DAP:

Sample letter

SEND OUT DATE: Monday November 10th

Many of us have been really busy this week. Take your time to write a letter over the weekend and send it out on Monday morning. This will also give us time to inform others who don’t frequent this site as often.

It sounds like sending them to his Chicago office is the best idea.
Here’s the contact info:

Chicago Office
John C. Kluczynski Federal Office Building
230 South Dearborn St.
Suite 3900 (39th floor)
Chicago, Illinois 60604

FAX/EMAIL DATE: Wednesday Nov.12 or Thursday Nov.13

We are trying to time this so letters and faxes will be received at the same time.

Fax:
(312) 886-3514

Email:
http://obama.senate.gov/contact/

Popularity: 2% [?]