Things to avoid
Here are some pitfalls to avoid. Don't:
Use a template letter, keeping the content the same and changing just the potential employer's name and address. You need to tailor each letter you write to the organisation and the vacancy on offer.
Begin each paragraph with "I", as this is too repetitive. Use an introductory phrase or "you" to add variety.
Go overboard about what you want. Instead focus on the criteria the employer is looking for, unless you are writing a speculative letter of application.
Be sloppy with your grammar and spelling.