Does Grassroots Lobbying Work?:

A Field Experiment Measuring the Effects of an e-Mail Lobbying Campaign on Legislative Behavior by Daniel E. Bergan from Michigan State University.

...I will post the full study on the website link text The study didn't say how to motivate more people to do it though!

Study conclusion:
Numerous grassroots campaigns are conducted by interest groups at the
state and national level each year. This article has described a simple
research design to estimate the effects of these efforts on public policy. The
results here suggest that the effect of e-mail campaigns on legislators is
substantial. This confirms the effect on policy of a fairly common form of
participation; 28% of the public claims to have contacted an election official
in the previous 5 years (General Social Survey 1972-2002), and many
of these contacts result from the mobilizing efforts of citizen groups
(Kollman, 1998).
Other methods of contacting legislators, such as phone campaigns, may
be more effective. This is suggested by polls that show that legislators pay
more attention to phone calls and personal visits than e-mails (e.g.,
Cornfield, 1999-2000). Such a result would also be consistent with research
on vote drives (e.g., Green & Gerber, 2004) that has shown that more
personal contacts with individuals, such as face-to-face contact, is far more
effective than less personal contacts, such as e-mail.