Keep it short and tailored to the job to which you are applying
Spelling. This is far and away the most common mistake. Carefully double check spellings of names and places. Some basic errors have a tendency to go unnoticed if the spellchecker results are approved. Copying and pasting the text into a fresh document will result in the spellchecker finding these sorts of errors. This is especially true on older resumes.
Grammar. Perhaps more noticeable than spelling errors as mistakes here can obscure the intended meaning. Reading text out loud is an effective way to eliminate unnecessary words that might otherwise go unnoticed. It is important that the language used makes sense while also conveying intent.
There is no need to be fancy. Odd fonts and colors, provided you aren’t in graphic design, will ensure a resume is not read by the people who need to see it. Keep it neat and clean, with simple and clear lists. Pictures and watermarks are unnecessary as are word-boxes.
Be professional. ‘Dress for the job you want,’ goes the old adage. This can be extended to the content of a resume. Only information applicable to the sought after position should be included. Employers will look for name and accurate contact info, an overview of experience, a list of critical and technical skills, a work history detailing the role, and a list of educational credentials, including alma mater and any certifications. Personal information such as likes, hobbies, and non-professional extracurricular activities are to be omitted.
Formatting should be simple. Bullet points and whitespace are the only necessary delimiters. Name and contact information (email address and phone number) centered at the top of the page, company names should be bold, as should explicit dates (month/year-month/year or present), titles italicized and underlined, and descriptions should be clear with listed duties given in bullets free from personal pronouns (e.g. I, me, my).
Uniformity is crucial. This is a simple rule. The way you refer to a company or activity is the way it should always be referred to. The formatting of all listed positions should be identical. Keep in mind that LinkedIn profiles function similarly to resumes. Names and dates given should be consistent between the two mediums. Also important to note is that industry-specific abbreviations should follow the explicit naming of that thing and should only be given if said abbreviation is used later (presided over the Server Maintenance Group (SMG)…cut SMG spending by 10%; however, terms such as LAN or SQL need no qualification).
Most importantly, keep it as brief as is possible. A concise and purely informative resume is useful and practical. Of course, a 30-year professional will have a considerably longer resume than a 5-year professional, but that is to be expected. Resumes need not be cluttered by every detail of work history or an exhaustive list of interests. Resumes are not intended to be personal; they are designed to get the attention of the hiring manager in an effort to get you, the candidate, an interview.