The hardest part can be waiting for a call back and decision. How long should I wait before calling or writing?

Before leaving the interview, you should say, “May I ask what sort of time-frame you’ve set for the decision?  I can follow-up with you based upon that, if it’s all right with you.”  Then, write and mail a thank you note the minute you get home from your interview, or no more than three days following. Keep in mind that decisions may be made more quickly than they’re indicated, so the sooner your note arrives, the better.