Well, there's a number of ways actually. You get an activity credit the more active you are on the forum. Posts and the amount of login time. Conversely, there's a small deduction for extended periods of inactivity, but not an egregious amount. Credits are also awarded on the basis of post length and word/character count, so longer posts earn you more points.
There's also the ability to request a loan from the bank, where you can pay back in payments every 30 days. Shortly we're going to start adding jackpots to the Arcade, where you can earn more credits by beating the high scores. Users can also purchase cards off of each other if they see something they like. Name that Tune also uses a Jackpot system and if I'm not mistaken, I thought Triad gave a reward at the end of a successful match.