In most cases, the person doing the hiring at whatever company you're applying to will have only your resume and maybe a cover letter to decide whether you are worth an interview. A cover letter can certainly help convince someone you deserve a chance, but a resume can easily show the person that you don't.
Your resume is more than just a list of your past jobs. It's a document that can tell the person receiving it a little bit about you as a person. That can be a good thing or a really bad one, depending upon whether you make the following mistakes.
1. Don't lie
This should go without saying, but it's tempting to put things on your resume that aren't quite true. If you claim a degree or a past job you never earned or never held, that will likely be discovered -- these days, everything is discovered.
You should also avoid small lies. For example, in college, a colleague of mine at the humor magazine listed "graphic design" as a skill on his resume. He got the job, but knew nothing about graphic design, and I was pressed into service teaching him over a very long weekend.
He got away with it because he learned the skill easily. But honesty would've served him better. Simply stating in his cover letter that he was computer-literate and willing to learn new skills would have accomplished the same objective, all while maintaining his integrity.
2. Don't be informal
The fact that you love cats, surfing, beer, or even heavier substances might be an interesting part of your personality. It should not, however, be part of your email address or any web page listed on your resume (or anything easily discoverable online). Use a formal email address consisting of some version of your name, or perhaps your name and a number if the easy variants of your name are taken.
3. Hire a copy editor
As a newspaper editor, I used to hire reporters and editors. If a resume or cover letter contained spelling errors or typos I generally put someone in the reject pile.
To be fair, the job I was hiring for required grammar skills. Still, your resume is essentially an open-book test. You're allowed to get professional help, and the person doing the hiring may question your judgment if you don't. Mistakes on a resume may make someone question your attention to detail, so hire a copy editor and make sure you check their work.
4. Don't be vague
It's great that you were a sales director who "increased sales." It's better if you're a sales director who "hired the entire team and doubled revenue in 18 months across a 35-state territory."
The person reading the resumes and deciding who to interview likely receives lots of qualified applicants. Yes, you have the right background, but were you good at the job? Show that you produced measurable -- and impactful -- results, and offer added insight as to what you might do if hired.
Go over every detail
Every line on your resume should serve a purpose. Read the document multiple times and make sure that every word on it works. When it comes to your resume there's no such thing as putting in too much time or being too diligent.
Remember that when applying for a job, your resume is the version of you the hiring person will meet first. Make sure that you put in the work required to make that initial impression a good one.