If you telephone, ask to speak with the staff person in charge of the issue about which you are calling. Explain your position and then ask for the representative's views on the issue. Be polite but persistent. Signing preprinted form letters is less effective than retyping them onto your letterhead. Ask for a written response.
Many offices accept e-mail, but written letters carry much more weight.
Try a personal visit. If you want to make a more substantial difference in how your representatives vote, make appointments with them or one of their staff members. Personal visits to your congressmen take more time, need more preparation and may seem daunting at first, but they can be marvelously effective. Telephone calls and letters generally get less attention, and responses can be delayed. Personal visits tend to rate highest priority, and their issues are attended to more quickly and more seriously.