A simple way to implement this is a filter at the receiving end which only allows email from known senders. There may still be some spam which slips through by pretending to be from known senders, but since each person has their own list of known senders, this is very difficult for the spammers to fake.
If you are running a web site, one way of allowing real users to contact you while still reducing the spam is to have a registration form on your site. This form would allow users to sign in and get their email address added to the list of recognized senders.
At present technology, a form like this is enough to stop the spammers. If the spambots (programs which scan websites looking for target addresses) get smart enough to actually submit a form like this, then the next defense is simply to send a registration confirmation to the registered email address and wait for the acknowledgement before adding the email address to the list of known senders.
This may be one of the reasons that more and more sites are already requiring that you register, for free, to use their site.