O2 replacement

I am helping a friend change the O2 sensors in his 2001 Silverado and need to know if there are any secrets to making this job easier? It also got me to thinking I should change mine, but I dont know how often they should be replaced. I take it I should buy OEM as opposed to the cheaper universals? Thanks!

if it ain't bad don't i wouln't bother changing them. there are 4 sensors and they are not cheap. most of the time a 7/8" wrench is all you need. the pcm should set codes if the sensor(s) are bad