At the end of your interview, if you haven't been advised, ask when they think they'll be making their decision. At least then you'll know how long you'll have to wait before you hear.
Many places don't automatically let people know if they haven't got the job; so one interview follow-up call is allowable. More than that and it can feel like badgering.
No matter how badly you think the interview went, if you want the job, always send a follow-up letter. Since most of us think of clever things to say after the fact, include one or two of those, referring to something specific from the interview.
Use phrases such as:
'I've given a lot of thought to our interview and...'
'Something you mentioned got me thinking...'
'What you said about _______ really struck home...'
If you don't get the job and you're curious why not, phone up and get some feedback. It may help you for the next interview.