Your resume is a marketing tool.
It speaks for you in your absence.
You don't get to follow your resume around and speak or explain anything. It gets passed from person to person through a hiring company's processes, so it must send the right messages on its own in order to generate the interest to interview you and perhaps hire you.
The resume by itself must get you through certain gatekeepers in order for you finally get the chance to speak for yourself.
how to generate a MARKETING MESSAGE
If possible, EACH section of your resume should state skills or experience that directly satisfies the job's requirements you are applying for. [Sections are those parts with titles (i.e., Objective, Summary, Skills, Education, Employment, etc.). For purposes here, consider each employer listed under Employment as also being a section.]
Any experience or skills that match that of the job requirements you are applying for should be listed FIRST within each section of your resume. This means that each section's first 1-3 sentences -or- each list of items within a sentence or summary -or the first 1-3 bullets within each collection of bullet points should include information that directly satisfies the job's requirements of the position you are applying for.
Just like a TV commercial, repeating those message multiple times and repeating them FIRST in each section will build your brand by implanting the idea over and over that you are a match for the position. What a person reads FIRST in a section will carry more weight than what is in the middle or the end. And seeing the same message time and time again will quiet their resistance that you are not a match and increase their confidence that you may or that you are a match for the position, thus making it more likely that you will be asked to the next step of their interview process.
Considering that you will apply for different jobs, you need to have a version of your resume that matches each general type of position you will be applying for. Many candidates amass skills that allow them to apply for 2-3 different type positions, so it's not uncommon for people searching for a new position to have 2-3 versions of their resume.
And it's not uncommon to make small changes to one of those versions for each position you apply for, tailoring it to each position's job requirements.
One resume does not fit all job requirements.
Shorten your resume
A resume should be 1-2 pages long, but it's not bad for it to be 3-4 pages. It should never be more than 4 pages (our opinion). People naturally think that the more content you ad showing all the great stuff you've done, the more impressive it is. But that's not the purpose of the resume. You will bore the heck out of your audience if it is too long and you will diminish your chances of being asked to participate in the next phase of the interview process.
You can have 100 years of experience, but still have a 4 page resume. Here's how:
The key is to realize that your last 2 years of experience is what an employer relies on the most to make the decision to interview or not. A potential employer is only mildly interested going back 3-5 years, and they are only curious in anything over 5 years old. You have to mentally let that time go and not be a 'packrat' when it comes to every piece of job history information you've ever recorded on your past resumes.
So...cut out the excruciatingly long details as you go back in time. Your resume should get less and less detailed the further back you go, to the point of only listing "company name, dates of employment, and title" at some point (say, over 10 years ago - and certainly over 20 years ago). Employers only want to know how you got from the start of your working career to where you are now. They do not care to read about the minute details of what you did "back in the day". It will not get you an interview and it will diminish your chances of getting interviews if the resume is too long.
Bottom line, no matter how many years of experience you have, it is possible to get your resume down to 4 pages or less.
Industry experience matters. Except for certain IT areas that tend to be the same across industries (networking, PC Support, some DBA work), industry experience becomes more important and "sticky" the more overall experience you get. Project Managers and Directors that are brought in from the outside almost always have the same industry experience as that of the hiring company. It gets harder and harder (and at some point impossible) to switch out of an industry as your career matures.
What's a good job history? If you are searching for permanent employment, ideally you want to show 2-3+ years per job. One or two shorter job stints won't have a significant impact, but more than that signals you are a "job hopper" and you will find yourself being ignored (not chosen) for interviews. The best predictor of future job performance is your past performance and if you constantly leave jobs before 2 years are up, then a potential employer will assume you will also leave his/her job within the same time frame. The #1 reason potential employers pass on a resume that shows the right skills is bad job history. Think twice before leaving a job sooner than 2-3 years into it. Career longevity will eventually depend on your individual job longevity.
Technical Skills "Sell"
Technical skills"sell" - for almost all IT jobs and at almost every level. Hiring authorities want to know the technical environment you did your work in and they want to see that information embedded in the duties/accomplishments sections by company/job. It's the backdrop to the work you did and it fills out the landscape of the experience and skills you present.
So...languages, operating systems, packages, methodologies should all be mentioned for each employer within the Employment section - by 99% of all people with an IT resume.