Mixer taps come with rubber or plastic gaskets.  Fit the gasket between the tap body and the bath to prevent water from seeping beneath the tap.  For plastic baths, fit a metal plate between the rear nuts and the underside of the bath rim to prevent strain on the plastic

Although there are other types of nonsetting sealants, slicone is the best for waterproofing the joint between walls and baths.  It comes in a variety of colours too.  The ones that contain a fungicide help prevent the growth of mould.

Where the gap between a bath and the wall exceeds 6mm, apply self-adhesive sealing strips around the bath.  This will prevent the nonsetting sealant from falling into the void time and time again.  To get the srtips to stick permanently, make sure you clean both the wall and bath very well.  Use both cleaning liquids and methylated spirit for cleaning.