The best method is to visit in person, either at their DC office or in their home state office. The second best is actually one that few take, but if there is a lobbyist for your interest, contact them. They will often have direct contact with not only your representatives, but others as well, and thus your concerns are passed on to many as opposed to a few. After these two, then calls, letters and email, in that order. Email isn't ignored, especially if there are lots of it, but phone calls tie up people longer and thus get more recognition.
No matter what method you use, always be polite, respectful and direct. Verbosity is not a plus. Their time, as well as that of their staff is important, so conciseness is important. State your point and reasoning and offer to make yourself available if they have any questions and let it go at that. Rambling, lengthy diatribes are generally written off and ignored.