- Proofread beyond just spelling and grammar. In particular, if you're interviewing with multiple organizations, make sure you're sending the right letter to the right organization.
- USe the follow-up letter to reiterate your best qualities. This a good chance to refocus the employer on what your comparative advantages are.
- Show off your listening skills. If you've taken good notes during the interview, you can match your follow up letters to the points mentioned in the interview.
- Tap into the employer's culture, but keep it professional.
- Write a separate letter to everyone you meet. Again, here's where taking notes is so important - these shouldn't be "boilerplate" - each letter should be individualized (they often get compared).
I'm always amazed how so many people fail to take care of the "little" things like good followup letters. They're actually not little things at all, because they have a major impact on the impression we make (and that's a huge part of getting the job).