An ordinary e-mail works the best. All of the Congressional members have official e-mail accounts which their staff monitor on a constant basis.
The contact pages on their web sites get attention too.
What the staffers are actually looking for is a message from a constituent requesting assistance with a federal department. The staff often has contacts in various departments and can resolve some problems with a phone call or two. If nothing else, the staff usually can give helpful advice.
Congressmen like to show off their clout in this manner. It doesn't really cost them anything and it builds a lot of good will.
Any other content in a message will get at most a polite form letter in reply. A critical or irate message usually will get no reply.
It is all the same really. Congressmen do not care what you think or what you want. They don't need to.
Good post.