Threads Magazine has a pretty extensive website with lots of interesting sewing info.
This Understanding Thread Tension article is a very nice overview of how machine tension works.
Towards the bottom of the article, there are two ways they discuss on how to adjust the tension.
I didn't have a clear understanding of the tension of the bobbin case, so this was a very helpful article for me.
The article also goes through the other things to check before adjusting the tension regulator or the tension screw on the bobbin case.
