Many people think that once an application, resume, cover letter, or set of references has been submitted, the situation is completely out of one's hands. They believe that they have done all they can by providing the necessary information and that they are now out-of-the-loop regarding whether their materials will be accepted or declined based solely on their appearance on the page. No matter how important those initial documents are, I have good news for you: this is not the end of the road. You have as many opportunities to continue contact with employers as you provide for yourself.
It is not always necessary to wait for a call before contacting an employer. The moment you see a job posting of interest, it is completely acceptable to contact the employer for additional information. Even something as simple as needing to find out the hiring partner's name is a good enough reason to call. The telephone interaction can be brief. Simply introduce yourself, explain your interest in the position, and ask for the information you would like to know. What is the name of the hiring partner? What department does this position fall under? What are the anticipated responsibilities (if they are not already outlined for you)? Any of these short and sweet questions provides an initial opportunity to get your name on people's minds and, perhaps, circulating around the office.
Upon submission of your cover letter and resume along with any other additional requested documents such as salary requirements or reference listings, you may want to contact the employer again to ensure that all the documents were, indeed, received and ended up in the right hands. Especially when documents have been mailed or faxed, there is a real chance they did not make it through all the correct channels or got lost in the mix of paperwork passed around the office. With this in mind, it is perfectly legitimate to request that someone check and make sure all the documents were received. This, once again, provides you an opportunity to introduce your name and reason for calling to another person in the office.
After an interview or any period of time spent in the office interacting with potential coworkers, it is considered good etiquette to send a thank-you letter via fax or email and perhaps even call to make sure it arrived. When you do this, you are expressing a continued interest in the position, the people involved in the hiring process, and the integrity of your work.
Any time an employer approaches an interaction with you with the assumption that he or she will contact you again later, make a mental note of the contact promised, provide a reasonable amount of time for a response, and then be sure to follow up yourself. Even if all you can say on the phone is that you wanted to see if any progress had been made toward filling the position, you are, yet again, making positive, professional contact with the people you might one day work with.
In general, you can utilize as many or as few of these opportunities to contact employers as you like, but each time you pick up the phone or send an email is just as important as the last. First impressions are everything, and you may speak to a brand-new person every time you attempt to contact the office. Keep this in mind, but also remember that you do not want to nag those involved in the hiring process. As long as you do not demand to speak to someone who is "not available" and do not attempt to harass anyone into providing more concrete answers to your questions, you should be in fine shape.