Really basic advice from someone who is hiring right now, albeit for really specialized jobs.
1. Spell check your resume
2. Tailor your resume for the company you are applying for. There's nothing more likely to get you ignored than saying you've always respected CostCo as a company, when you are actually writing to Wal-Mart
3. Do not leave the edits in from the previous application letter. Always cut and paste what you are writing into a new document so the "show edits" feature of Microsoft Word does not show the edits.
4. Keep that letter short and relevant. Don't ramble on over 2-3 pages.
5. If you get an interview, show up on time, and looking professional.
6. To paraphrase" Ask not what the job can do for you, tell me clearly what you can do for the job. I am always amazed at otherwise qualified candidates who damn themselves when they start an interview saying "this job will be good for me because I need the experience" rather than "I will be good for this job because I have the experience."