Firstly, do you absolutely have to? It is a potentially dangerous thing to do. There was a guy in town who had to be rushed to hospital with a severed artery when smashing up a piano a couple of years back and the risks are the same as from piano strings.
If there is no alternative, helmet, visor, earmuffs, gloves and protective overalls are essential. Also make sure there is nothing in the room downstairs which can vibrate loose (pictures, ornaments etc).
The best place to start is along the edge at the middle of the sides, hitting from the inside. Try to split the bath in half across the middle. Obviously a weak point is the overflow and waste hole, hit from the inside.