There is a prevailing belief among many job seekers that a resume is simply a repository of all work experience ever obtained, no matter how old, how unrelated, or how meaningless. Just throw it all in there and call it keyword richness. Let the recruiter figure it out. It's one reason that so many self-written resumes don't work. I call it the data dump method.
The mentality behind the data dump is usually good intentions. The job seeker knows he/she should get the important information in the resume but isn't sure exactly what is important and what isn't. To the job seeker, everything is important and has some sort of emotional connection. Not mention the fact that you were the winner of the state spelling bee in ninth grade twenty-five years ago? Unbelievable! That was a big moment in your life! In the interest of safety, the job seeker just includes everything and hopes for the best, hanging onto past trophies for dear life. Unwittingly, by throwing everything in there he has just killed the resume. Recruiters do not have time to wade through tons of information in search of nuggets of gold. They need the key information right up front and speaking to their needs.
To avoid a data dump in your resume, you must start with a focus. One of the underlying causes of too much information, or scattered approach in the resume is lack of keen focus. Do not try to be everything to everyone. I've seen resumes with areas of expertise listed that stretched to ten or fifteen different items, many not related. Resumes such as this leave the reader confused and gives the impression the job seeker is also confused. Employers are looking for problem-solvers with experience solving problems that are similar to the ones they currently face. They don't hire people who MAY be able to solve their problems - they are looking for specialists. Employers don't hire jacks of all trades so don't try to be one in your resume.
After you find your focus, you need to very critically examine the content of the resume, and perhaps content that you originally did not include in order to identify the pieces of your experience that support your focus. This is called strategy. Most job seekers who write their own resumes don't think of strategy as a key part of a resume but rather concern themselves primarily with mechanics and format. A professional resume writer starts thinking strategy immediately - How can I position my client's experience to speak to the employer's needs? What information will support the client's goals, support the targeted salary level, and portray the client as the answer to the hiring manager's prayers?
A resume must be built around the strategy. Sometimes, a job seeker isn't sure what he/she wants to do - maybe stay in the same type of job, maybe find something in the same industry, or maybe go in a completely new direction. The indecision makes it impossible to write an effective resume because there is not a focus around which to build the strategy. No strategy results in a weak document.
Do you have a resume that's four pages long? More than likely you need to find your focus and start working on your strategy.