Swarfega (sp?) the best, polish a small dab onto the inside of the visor - make sure you don't leave smear marks which mess up your vision!
Second best - Fairy Liquid, just choose the best smelling one for you and polish a small drop onto the visor.
Works for about 2 hours of continuous use before reapplying.
The old ones are always the best/cheapest/most stupid/unfashionable (delete as appropriate
)