List your technical knowledge first, in an organized way. Your technical strengths must stand out clearly at the beginning of your resume. Ultimately, your resume is going to be read by a thoughtful human being, but before it gets to that point it often has to be categorized by an administrative clerk, and make its way past various sorts of key word searches. Therefore, you should list as many directly relevant buzz words as you can which reflect your knowledge and experience. List all operating systems and UNIX flavors you know. List all programming languages and platforms with which you're experienced. List all software you are skilled with. Make it obvious at a glance where your strengths lie - whether the glance is from a hiring manager, a clerk, or a machine.
List your qualifications in order of relevance, from most to least. Only list your degree and educational qualifications first if they are truly relevant to the job for which you are applying. If you've already done what you want to do in a new job, by all means, list it first, even if it wasn't your most recent job. Abandon any strict adherence to a chronological ordering of your experience.
Quantify your experience wherever possible. Cite numerical figures, such as monetary budgets/funds saved, time periods/efficiency improved, lines of code written/debugged, numbers of machines administered/fixed, etc. which demonstrate progress or accomplishments due directly to your work.
Begin sentences with action verbs. Portray yourself as someone who is active, uses their brain, and gets things done. Stick with the past tense, even for descriptions of currently held positions, to avoid confusion.
Don't sell yourself short. This is by far the biggest mistake of all resumes, technical and otherwise. Your experiences are worthy for review by hiring managers. Treat your resume as an advertisement for you. Be sure to thoroughly "sell" yourself by highlighting all of your strengths. If you've got a valuable asset which doesn't seem to fit into any existing components of your resume, list it anyway as its own resume segment.
Be concise. As a rule of thumb, resumes reflecting five years or less experience should fit on one page. More extensive experience can justify usage of a second page. Consider three pages (about 15 years or more experience) an absolute limit. Avoid lengthy descriptions of whole projects of which you were only a part. Consolidate action verbs where one task or responsibility encompasses other tasks and duties. Minimize usage of articles (the, an, a) and never use "I" or other pronouns to identify yourself.
Omit needless items. Leave all these things off your resume: social security number, marital status, health, citizenship, age, scholarships, irrelevant awards, irrelevant associations and memberships, irrelevant publications, irrelevant recreational activities, a second mailing address ("permanent address" is confusing and never used), references, reference of references ("available upon request"), travel history, previous pay rates, previous supervisor names, and components of your name which you really never use (i.e. middle names).
Have a trusted friend review your resume. Be sure to pick someone who is attentive to details, can effectively critique your writing, and will give an honest and objective opinion. Seriously consider their advice. Get a third and fourth opinion if you can.
Proofread, proofread, proofread. Be sure to catch all spelling errors, grammatical weaknesses, unusual punctuation, and inconsistent capitalizations. Proofread it numerous times over at least two days to allow a fresh eye to catch any hidden mistakes.
Laser print it on plain, white paper. Handwriting, typing, dot matrix printing, and even ink jet printing look pretty cheesy. Stick with laser prints. Don't waste your money on special bond paper, matching envelopes, or any color deviances away from plain white. Your resume will be photocopied, faxed, and scanned numerous times, defeating any special paper efforts, assuming your original resume doesn't first end up in the circular file.