One of the most unfortunate and unprofessional mistakes job candidates make is writing a poor description of their work experience. Phrases that begin with "I was responsible for . . ." and "My main duties . . ." are a sure way to get your job application rejected.
Avoid making the mistake of simply listing duties, which basically states only what tasks your former positions encompassed. Step it up by showing what you accomplished at those jobs.
In short, a duty describes what you did; an accomplishment describes how well you did it. You should only list a task when you can prove how good you are at it.