"Ben Poras Yosef, Ben Poras Alei Ayin" [Vayechi 49:22]. Rashi says that this means that Yosef will multiply and be beyond the reach of Ayin Hara [the evil eye]. As a reward for not taking his master's wife, no one will be able, through jealously, to inflict any harm on what belongs to him.
The Shulchan Gavo'a brings from Rav Eliyahu Dessler that no matter how rich a person is, no one is ever jealous of a totally selfless person whose whole life is about giving. An element of jealousy stems from the intended or even unintended flaunting of oneself before others.
Yaakov gave Ephraim and Menashe a bracha "V'Yidgu LaRov" - they should multiply like fish. There are two attributes of fish that Yaakov had in mind. Fish are not seen from the dry land. Moreover the fish live a life totally separated from the inhabitants of the land. They don't compete with them in any way. That is why the Ayin Hara does not affect them.
If a person lives a life of Yosef, where he doesn't want what doesn't belong to him, and he lives and enjoys his material assets out of the public eye, he too will not suffer from any unwanted evil eyes.
Source: Rav Eliyahu Dessler