If I have an image that I like, but the crop doesn't seem obvious, I usually start as @brett8883 mentions and use established rules of composition, but I won't hesitate to deviate from those norms if my eyes tell me otherwise.
I can see your...
Do you have a process you follow for cropping?
Or do you just play with it until it pleases you?
Here are three from the same large pano:
And another two from a different pano of the same viaduct:
Which would you keep of each batch...