This site is an excellent resource for activities and tools to use to help your parish become active in social justice issues.
Letters written by hand are less likely to be form letters and show a commitment of time.
Of course, typed letters are also effective. If you're able to write directly to the staff person working on the issue of concern to you, you may have better luck in receiving a letter of response, especially if you have talked to them on the phone previously. Office fax machines are usually swamped but sending a fax is an option for last-minute contact. If you have the personal fax of a staff person, rather than the general office number, your fax will be sure to be seen. Visiting someone in your Congressperson's office is a great opportunity to talk about an issue of concern to you.