What to include in a cover letter

Try to limit your letter to a single page. Be succinct.
Assess the employer's needs and your skills. Then try to match them in the letter in a way that will appeal to the employer's self-interest.
As much as possible, tailor your letter to each job opportunity. Demonstrate, if possible, some knowledge of the organization to which you are applying.
Write in a style that is mature but clear; avoid long and intricate sentences and paragraphs; avoid jargon. Use action verbs and the active voice; convey confidence, optimism, and enthusiasm coupled with respect and professionalism.
Show some personality, but avoid hard-sell, gimmicky, or unorthodox letters. Start fast; attract interest immediately. For more information see Business Letter Format.
Arrange the points in a logical sequence; organize each paragraph around a main point.