Whenever possible, avoid the use of mundane form letters - they are very boring and easily recognized by employers. Develop a smooth style to your letter-writing that is easy and enjoyable to read. The key is to be optimistic as well as accurate. Be sure to use clear, direct language, and avoid cliches. Use quality bond paper - preferably, use the same paper for your letter as you do for your resume.
Laser printer is preferred.
Some inkjet printers are adequate, but not recommended. White is preferred for electronic application. Keep it neat. A letter that is sloppy or riddled with obvious mistakes is sure to reflect negatively on your competence. Be certain that your letter is clean, neat, and error free.