I will admit expressions are my downfall when it comes to After Effects. It doesn't help that anything dealing with numbers/math are a foreign language to me.
Anyway I need to make a still image of a dangling earring rotate back and forth on an ear. I tried wiggle and other expressions but was unsuccessful. I then created the effect manually with rotation keyframes. I am sure there is an easier way to do this especially since the image will be in the FG and blurred so the movement does not need to be exact just the illusion is enough.
I will need to also start and stop the movement which I think could be applied to the right expression with a slider control and just keyframing to 0.
I usually just use Ease & Whiz elastic expression for something that needs to dangle and settle into place. The key is the time between keyframes and "period for elastic" parameter in the expression. You can fiddle with the "amplitude for elastic" and that sometimes will help get hte look your going for, but mostly just the keyframe distance and "period for elastic" is what I adjust.http://aescripts.com/ease-and-wizz/