I have been told a million times I should help people write resumes and cover letters for a living. Interviewers have complimented me over and over on them, even if I didn't get the job. I often do others' resumes for them.
Make your resume perfect- that's it. A simple design, with some punch helps, but your resume needs to be completely free of typos, spelling errors, be grammatically correct, have proper punctuation, etc.
Do your research on the company before doing the cover letter. Always address it to a particular person - if the ad doesn't say who that person is, call the company and find out. Have a cover letter that is clear, precise and explains exactly how your background, skills and qualifications match their requirements. Use the cover letter as an opportunity to showcase who you are, but never forget who the employer is and what they need. Change your cover letter for each company- this is not the time to save time by being generic.
Have someone check your resume and cover letter over carefully, not just for proofreading, but for content and to ensure that you are really highlighting what's important and perhaps de-emphasizing your less than great areas.