There are a lot of ways to gain credits but also some ways to lose them. When you receive an infraction or when a post you wrote gets deleted, a penalty may be applied.
The interest is currently set to gain monthly. We're going to need some time to see how the economy is doing before we consider changing that time. The other features we have actually make it easier to gain credits faster than before. And frankly, we'd prefer to reward people credits for doing things like posting or starting threads than for putting their credits into savings and letting it sit.
The numbers under your post counts are the amounts in your savings. The Credits plug-in by default has your checking account balance displayed there. There was overwhelming support for changing that to savings which, unfortunately, don't get rounded the same way. I'm working on a fix for that.
Loans are now enabled. Users may apply for a loan of up to 1000 credits per loan, and you can have up to 3 outstanding loans at at time. Interest is 19.5% however, so if you're gearing up to lay a big bet on UFC 79, you better hope you win or you'll have a lot of interest to pay.