Be sure to look for any typos, misspelled words, grammatical errors, and punctuation errors. These can make an application letter appear sloppy and unprofessional. Even though you discussed your skills, experience, and qualifications in the letter, you should still send a resume with your letter. This will let the hiring party know the details of your work history and education, with specifics like the duration of a given job or volunteer service. Attaching a resume also shows a certain amount of professionalism on your part. It lets the hiring party know that you take the position seriously, and are willing to work for the opportunity to be a part of the organization. Though not every internship requires letters of recommendation, some do request them. Even if they are not required, including letters of recommendation can be another way to demonstrate your commitment to professionalism.  Letters of recommendation should be written by professional or personal references who can attest to your character and capabilities.  In lieu of letters of recommendation, you may want to include a list of professional references with each reference's contact information. This will allow the hiring party to reach out to your references, if he or she needs to, and it shows that you are able to maintain positive working relationships with current and former employers and/or colleagues. The organization should specify whether letters are to be submitted online or as a hard copy (which can be sent by mail or dropped of in person). Follow the organization's requirements. If sending your letter by mail, be sure that you have affixed the correct amount of postage to your letter, and that you have the correct contact person's name and mailing address for the envelope. Wait at least a few days (perhaps even a week) before following up, and don't be too pushy or demanding. Simply send a cordial email or place a call during business hours to the contact person for that volunteer position. Let him or her know that you've sent in an application letter and that you wanted to formally express your interest in the opportunity to be a part of the organization. And above all else, remember to be professional and courteous. This is a polite and professional way to let the hiring party know that you appreciate his or her time. The thank you note can be an actual letter or an email, and you should send a note to each interviewer you met with.  Address the interviewer by name.  Thank him or her for the opportunity to meet and discuss the position.  Try to mention something specific that the interviewer talked about during your meeting. Letting him or her know in the thank you note that you were interested in what he/she said shows that you were paying attention and that you take the opportunity seriously.  Close with a polite ending that does not assume that you have earned the position. Say something like, "I look forward to discussing this opportunity further," or wish the interviewer luck with the selection process. Some organizations are inundated with volunteer requests and may ask that you do not follow up. In each case, do as instructed.

Summary:
Proofread your letter. Include a resume. Arrange for two letters of recommendation. Submit your letter. Give a follow up phone call or email. Write a thank you note after any interviews you have.