Avoid anything that mentions the 80's or earlier. That means strip the dates off your qualifications and avoid listing your jobs back that far. Ideally I don't want to see anything from the 90's either but so many of my clients want it included that sometimes I have to relent.
Ditto for anything that shows your age - some really good examples are things like Typewriting or Shorthand courses. These are a dead give-away. The same as anything that mentions the word Secretary School. They don't call it that anymore so don't mention it.
Please highlight your achievements and don't be modest! This sounds odd (and also sad) but modesty is not something the younger generation are good at. So many of my mature clients want to tone down their resume because they are afraid of sounding conceited. Don't be - you are competing with a younger generation that is VERY comfortable shouting how awesome they are from the rooftop. You have to be in it to win it and that means you need to compete to the very best of your ability. Show them how great you are!
That's the main tips - I talk more about hiding your age on your resume to avoid discrimination in the video so give it a watch and get in touch if you have any questions.