Answer:
Sympathetic Nervous System:
If you think of it as "fight or flight" it becomes a lot easier to remember how it works. It increases the heart rate, increases energy production, decreases the GI system function, etc. This is to prepare you for immediate action.
Parasympathetic Nervous System:
It acts opposite to the sympathetic nervous system. So think of "rest and digest". It promotes GI system, it decreases heart rate, makes you want to visit the restroom, etc.