Think about how an administrator determines who should get interviewed. She's advertised the position in all the usual places. Every business day she receives dozens of envelopes from applicants. Why should your application go on the stack of applications marked 'interview' instead of the one marked 'reject'? Because when she scans your application, she sees most of what she is looking for. Remember: she's a very busy person. A lot is riding on her choosing the best candidate for the position which she has to fill.
1. Poor Initial Impression
Never fold your cover letter or letter of interest and the required supporting documents. Always insert unfolded materials into a manilla envelope. Use a paper clip to keep documents together. No stapling please. On the bottom of your pile of documents put a piece of cardboard 8.5" x 11 " to stiffen the envelope so that it makes it through the delivery process unscathed. Use printed labels for the address and your return address. You may think that your penmanship is impeccable. Be that as it may, print the labels using a printer.
2. Format Specified Not Used
Nothing gives the wrong impression of you and your abilities quicker than your failure to follow instructions. If the application instructions specify that your cover letter is to printed on pink paper, then so be it. You are asking for the opportunity to be part of a team. Members of a team follow the coach's instructions. If you cannot follow the simplest of instructions regarding the application procedure and other applicants can, then why should they consider your application?
Nothing is worse than a typo. It says bluntly that you don't care. Carelessness and sloppiness are not traits most administrators want to see in their teachers. If this sounds harsh, consider that dozens of other applicants will have submitted materials without blemish.
Even if you are a graduate of the school in question, don't presume to be familiar. The school will know that you are an alumnus. Besides that, you will have already spoken with administration about the position anyway. Your formal application must be just that: formal.
5. Being Long-winded
Administrators really don't have much time to read everything. They will probably simply scan your materials. Scanning involves looking at things which catch their eye. Like typos or long, runon sentences. When you begin writing your answers to questions on the application, always do a rough draft. Then put it away for a day or two. Come back to it with a fresh perspective and revise, revise, revise. Pare your answers to the bone.