How to prevent finger friction when using touchscreen for a long time?

Definitely the most commons solution to this is to use a screen protector. Take a look on amazon, or just go to your local phone store or mall and pick one up. I tend to prefer the ones that are more of a matte type of material.

The problem is actually caused by the oils on your fingers causing friction with the screen. Usually wiping your phone's screen off with a damp cloth, or some screen cleaning solution will do the trick. Make sure when you are cleaning it to not get any of the liquid inside any of the holes on your phone. If you do, remove the battery, and place the phone in a cup of rice for 24 hours. Also, after wiping your phone down, wash your hands and dry them thoroughly. This should remove all of the friction-causing oil from your hands/phone.

I would use a nice lint-free cloth to wipe down the screen whenever it starts getting gummed up.

Other than that, you might try using lighter pressure. Modern touch-screens are very sensitive and using lighter pressure will keep oils off your screen while also reducing friction.