FogCity is the leader in anti-fog inserts. They also come in a variety of tints.

The Fog City Anti Fog inserts are a must for any visor that did not come with an anti-fog coating from the factory.  In many cases the Fog City inserts do a better job than the factory coatings do.  They are quick and easy to install.  It shouldn't take more than about a minute once you center the insert on the shield.  After that viola!  A truley fog-resistant visor. 

The inserts should last for years if taken care of properly.  We have personally seen the Fog City inserts provide fog free riding for years!  In fact, they last longer than many of the helmets we have crashed.  We did notice one insert we had installed seemed to fog a little after about a year and a half of use, but we have have been a little rough on cleaning the visor, so we have have been at fault.  But 99% of the time the inserts really impressed our staff on their personal helmets.