You don't have to worry about writing great sentences yet or narrowing your list. Inventory your accomplishments. Now, go back to each job and think about what you might have done above and beyond the call of duty. Write down any accomplishments that show potential employers what you have done in the past, which translates into what you might be able to do for them in the future. Quantify your accomplishments whenever possible.
Numbers are always impressive.
Make appropriate deletions. Now that you have the words on paper, go back to each list and think about which items are relevant to your target job. Remember, your resume is just an enticer, a way to get your foot in the door. It isn't intended to be all-inclusive. You can choose to go back only as far as your jobs relate to your present objective.