Follow-up is very important. A thank-you letter, that restates your interest is essential. It lets the employer know that you are still interested in the position and that you would be open to an offer (no one likes to get turned down). A hand-written thank-you note should be mailed within 48 hours of your visit. Take no further action until at least one week beyond the date when you were told you would hear from the company. At that time, a phone call to see if a decision has been made is appropriate.
E-mailing a thank-you letter is appropriate at times. While a written note is the best choice, an acknowledgement of any kind is better than none at all.