Simply follow-up! This is just as important as your letter. A quick call to verify the arrival of your materials keeps you in control of your job search and provides an opportunity to learn more about the position and hiring process. Allow three to five business days after mailing the materials before calling. This is long enough to ensure they received and reviewed your materials, yet short enough so that they haven't totally forgotten about you. 

Unfortunately, many employers do not respond to every person who submits a resume and letter — try not to take it personally.