There was a recent diarythat talked about freeping the US Chamber of Congress. It had some good information on some anti-union bills along with some good wording on a letter written in opposition to the bills. Unfortunately, the diary also suggested using the Chamber of Commerce site to send the letter.
There was another diary that warned people that their message might not get to their representatives since the Chamber of Commerce site might block their messages.
I decided it would be useful to list a few links that I use when I'm researching a bill or writing to my members of congress.
Project Vote Smart has information on current office holders in state and Federal races. They try to get information on candidates in Federal races and some state races. They compile ratings from various groups for both state and federal office holders.
Thomas is a library of Congress site that has information about pending Federal legislation. This is helpful if you want to know if your members of congress have cosponsored certain legislation. As well as to get the actual text of a bill.
There are lots of groups that allow you to write to Congress (e.g. Union of Concerned Scientists, ACLU, etc). In general, you probably don't want to use a site that you disagree with to contact your member of congress because someone from that group can look at the message that you send. If they don't like it, they can block or send for pre-approval any messages you send.
I like the one from the Friends Committee on National Legislation (Quakers), in part because they have an easy way to send to several people at once. So I might send something to both of my senators.
There have been multiple studies showing how member of congress are influenced. Most have a particular mind-set, which is very difficult to change. But on issues that they are not conversant in, issues they don't consider important, and in general issues where they don't have a strong position, they can be influenced.
Constituents with knowledge about an issue area are valued, and that's where writing can be very useful. Lettes from non-constituents are generally discarded unread, while constituent letters are definately taken into account. Although lobbyists from non-profits have influence, their influence is often limited to how many people from that members district they represent.
I'll try to update this later, but lunch is over, so I have to get to work.