If you have really impressed, most employers will try and let you know that you've been successful as soon as possible - they don't want you to be snapped up by somebody else.  If you didn't, they'll often leave it a week or so and let HR tell you that you failed.  Here are some things to consider for after the interview:

• Call or write to their HR person or recruitment agency saying you were impressed with the company
• If you feel nervous after the interview, remember that it is common as human beings to focus on the things that went wrong rather than the things that went well.  Try and remember the interview objectively.
• If you fail, don't be too shy to ask why - it will help you in your continuing job hunt.
• If you succeed, remember that you don't have to accept their first salary offer.  The hiring process is arduous and costly for companies so don't think that they won't give you more money.  This is especially true if you managed to avoid costly agency fees for them.  Be prepared to produce a counter offer and repeat how much you loved meeting them in the interview.

Check out our Salary Negotiation Techniques