If there's something you had wished you'd shared during the interview, do it now. Mention anything you wished you had said, but didn't, during the interview.
☛ Don't harass the company.
☛ Don't go to the company in person, unless invited.
☛ Don't avoid the follow-up letter or phone call.
Even if you didn't get the job, thank the interviewer for their time.
Don't be angry or overly-disappointed - accept the fact that there were probably many other qualified candidates and view it as a valuable learning experience. Whether you are informed via phone or email, you should send a follow-up message to your interviewer thanking them for their time and for giving you the opportunity to interview with them.
Reply using the same method as they used to contact you - if they called, ring them back as soon as possible, if they sent an email, an email reply should suffice.
Making them wait for a response is unprofessional and makes it seem like you are not enthusiastic about the job. Don't be worried about coming across as over-eager - they want candidates who are excited by the prospect of working for their company.
Keep it brief and breezy, you could say something like:
☛ I hope the hiring process is going well for the position of Marketing Assistant. I believe you mentioned that a decision would be reached by Monday and I'm eager to find out if you have any information on the status of my application? Please let me know if I can provide any further information that would help you with the decision making process.
☛ If you decide to phone, make the call from a quiet place a few days after the interview. Pick a good time of day -- not right after lunch, early in the morning or at the end of the working day. This will maximize your chances of actually speaking to the interviewer.
☛ Be as polite and brief on the phone as possible - remember that you may be talking to a stressed-out hiring manager that has a 101 things on his/her mind other than the status of your job application. Don't call a second time unless you are specifically told.
After the indicated time period has passed, follow up with a single email or phone call. If the time period they specified it would take for them to reach a decision has passed - whether it has been two days or two weeks - it is now appropriate for you to follow up about the decision via email or phone call.
☛ Use the interviewer's first name in the greeting and salutation only if you were told to do so during the interview; otherwise, address the interviewer in a formal manner.
☛ After thanking the interviewer again for the opportunity to interview with the company, affirm your interest in the position and reiterate how you would be an asset to the company.
☛ Add information that the interviewer might be interested in, or some useful information that the company could use profitably. This will help the interviewer to remember you, as most people follow up after a job interview with only information about themselves.
☛ Close the letter with the valediction "Yours sincerely" and proofread it thoroughly for grammar, spelling and punctuation. A poorly-worded or misspelled thank-you letter can be all it takes to disqualify you as a candidate.
Mail a more formal thank you letter.
This can also be done via email, or through a typed letter, depending on the kind of company you are applying to. A social media related or tech company might appreciate the ease and efficiency of an email, whereas a company with more old-fashioned or traditional values would appreciate a letter. Whatever form you use, the purpose of the letter is to remind the interviewer of what a strong candidate you are and why you are uniquely qualified for the job. If you interviewed by multiple people, you should send each of them a separate thank you letter.
If you had a casual conversation with the interviewer before or after the interview and something came up about a particular restaurant or upcoming music event, request to share a link providing additional details.
The bottom line is that you need to request to connect on LinkedIn by making it interesting or worthwhile for them - it shouldn't look like a purely self-serving move.