Obviously not everyone can make it to DC for the march, organized by United for Peace, but Chris Bowers of MyDD provides alternatives:

Whatever the cause, there is more than one-way to march on Washington. MoveOn.org is sponsoring a virtual march on Washington on February 1st, which happens to be Thursday. This march will be made in the form of phone calls, emails and faxes, to send one million direct contacts to member of Congress to stop the escalation. No matter whether you attend the march on Saturday or not, no matter whether you were able to attend the march on Saturday or not, everyone can take part in this march. If, for one day, no one in Congress received any other messages from constituents except a call to stop Bush's escalation in Iraq, we can be louder and clearer than any mass rally. You can take part here.