What makes writing a resume difficult is that there are two audiences. First is the low-paid, non-technical HR clerk that receives the resume. If it gets past the clerk, it will arrive at the desk of the person that will be your future boss. Your resume has to have the elements that will please both of these people.

• The HR clerk - The first person to see your resume, sadly, is a non-technical, (usually low-paid), clerk. He is given 10,000 resumes a day and a list of what positions are open. This person then has to make a pile for each of those positions, and toss the rest. Your job is to make sure you get into one of those piles. The problem here is that this person doesn't know the difference between UNIX and Solaris, or that if someone knows Solaris 2.5 then they are hirable for a Solaris 2.6 job. Luckily, this person only reads the top part of every resume, so you can play to that: make sure that you have an Objective and a "Skills" section that are customized for him. Don't say "Solaris 2.6", say "Solaris 2.x" or just "Solaris" (people have forgotten about Solaris 1.x by now).
• The Hiring Manager - Each pile that the clerk created is handed to an appropriate "Hiring Manager." This person does understand the technology (or at least thinks they do) that you'll be working with. So the rest of the resume must be in their language.

The most common mistake that I see is that people don't write anything for the clerk. Therefore, their resume never gets to the hiring manager. The "Objective Statement" at the top of your resume is what the clerk reads. Make sure you resume has one, and make it a good one.