(Written for a friend who was teaching a class of young people.)
Do be selective. You don't have to list every job you've ever had. Highlight the relevant ones, and highlight specific things you learned or did at those jobs.
The purpose of a résumé is to get you an interview, not tell me your life story.
Never send a ten page résumé, or a seven page one, or even a five page one. If you can't prioritise what to put on a résumé, you're showing that you don't have that skill, which is really useful in many jobs.
No-one should need any more than three pages at the very longest for someone with decades of experience. Don't make your résumé a wall of text, give me enough to get my attention and move on. Hit the highlights and the most relevant experience.
If you're applying to be an X, make sure your resume mentions being an X, or training as an X, or research into being an X. Never apply for a job as an X and send a résumé telling me you're a Y.
Read the résumé out loud before you send it. When you read it aloud, odd sentence structure or awkward wording is a lot more obvious. Have someone else read it to you.
Don't inflate your skills. If you say you're an expert at X but you can't answer basic questions about X, then that makes me wonder what else is untrue on your résumé. Don't lie, exaggerate, or make stuff up.
There should not be ANY typos, spelling errors, or grammatical errors in your résumé. Print it out and red pen it. Then get someone else to red pen it.
Stick with basic fonts: Times, Georgia, Ariel, Helvetica. Stay away from Comic Sans and Papyrus. Don't use clip art, or colours. If your résumé gets printed, it'll almost always be on a black and white laser printer, colours make it pale and harder to read.
Make sure your contact details on the résumé are correct, it's the only way to get hold of you. If you set up a separate email address for the résumé, check it multiple times a day.
Don't belittle your past employers on your résumé. We know you want to leave, but bad-mouthing them on paper makes me wonder what you'll say about my company later.
If you have gaps in your employment history, be prepared to explain why.
If you're applying for an entry level job, research what the skills are for someone in that job, and showcase those in your résumé. Even if you don't have the experience, it shows you did the research and know what you're looking for.