Address your letter to a specific person - the person with the power to hire you. Get their name and title correct. Tailor your letter to the requirement of the position and the employer's needs. Send an original letter, not a copy. Write each letter in your own words, not something you cribbed out of a book.
Canned letters get canned responses or no response at all.
Than have someone else proofread it for you. Do NOT depend on spell check and grammar check on the computer. Drop it off in Career Services for a critique.