Originally posted by Kano
Many ways.
Primarily by posting. Some forums are worth more than others. (ie BTS-type forums are worth less).
By having people reply to threads you started.
By getting 'applauded' by staff for an especially good post.
By winning competitions (ie debates etc).
By filling out some surveys.
Theres probably other ways that I've forgotten.
One thing to remember though.
If you have to have a post deleted, you lose a lot more than what you gained for posting it.
The opposite of an applaud is a warn. If a staff member has to warn you you lose a big chunk of points.
So basically you gain points by being active and a good contributor to the boards. People who spam to try and gain points usually end up in the -ve.

you dont want to be warned.
you lose too much points.
like me