For those with a lot of duties that could be listed sit down and try to narrow your list to the top five that really showcase your abilities. Sometimes you can incorporate a few duties into one instead of listing each separately. For example if you supervise 40 employees and do their payroll and write their reviews that can all be worked into one duty under supervision rather than listing them one by one. That is just an example but hopefully you get the idea.
And for other individuals that have the opposite problem of not enough duties to list you would be surprised at what you do without even thinking about it. I went to meet with a client today and once we got talking there were a lot of tasks they had performed on the job that they took for granted but showed great leadership and management skills.
My best suggestion is to outline your resume and then fill it in and add and subtract until it is concise and to the point and less than two pages.