How to polish curving edges? I have 2 questions please:
1. Just wondering how to polish these edges and make it look clean? And make it look more like hard edges.
2. I'm not satisfied with how the curve looks. I'm using snake tools to create it.





Curves that I wanted to create
The Grab brush (Shortcut "G") is great for getting the right shape for your curve.

What I'd use here would be the Fill/Deepen brush (shortcut "5") and the Flatten brush (shortcut "6"). Then I'd gently smooth the flat surfaces until close to the hard edge (shortcut "Shift"). 

If there's any edge that I was completely happy with, the pinch brush (Shortcut "P") is great for getting a sharper edge to finish it off.

Just play around with strength and keep undoing things if you make mistakes.

Sculpting these harder surfaces takes a bit of practise but it's generally the fastest way.

OR

If you don't feel comfortable with this, perhaps you could try retopologising it using a shrink wrap modifier and a vertex group. This way you can remove certain vertices from the shrinkwrap vertex group and play with a specific vertex until you're happy. With your retopologised mesh, you can play with the Mean Crease in properties panel ("N") underneath the transform data to get those edges nice a crisp when applying a subdivision surface.