A cover letter should introduce your resume. It should be clear and concise without going into the kind of detail used in the resume. At the same time, it should give employers a little meat to chew on. One way I do this is to include a sentence that highlights skills that will interest the employer.
Here are a few examples followed by the kind of job sought in parenthesis.
My duties have included maintaining schedules/calendars, travel arrangements, correspondence, and meeting planning. (Executive Assistant)
My duties included vendor management, negotiation, inventory control, and coordination of delivery and special orders. (Purchasing)
My duties have included store operations, event sales, recruiting, and training. (Retail Manager)
My duties have included all aspects of classroom instruction as well as extracurricular activities that encourage academic and personal development. (Teacher)
These are just a few examples of how a set of skills can be packaged in one sentence. Using this kind of sentence is one way you can keep you cover letter specific and concise.