Help Me Get a Job

Do you have any tips for someone who is applying for a job and has been asked to write a cover letter or a personal statement? If you were that person, would you keep the letter brief and to the point, or long and descriptive? Would you try to mention all the skills and qualities they want or would you go for a more general description of your talents (as long as it covers what they are mainly looking for?) Formal or slightly informal (friendly)? Does it depend on the field you work in (as in my case, the Arts?) Do you think a person's age matters and therefore should be left out if possible?

Any tips would be very welcome! Thanks in advance.