surface area

air resistance

descent velocity

gravity 

more friction decreases speed

more friction increases speed

less friction decreases speed

friction has no effect on speed

Brakes used on a bike

Tires from a car on the road

Tread on the bottom of your shoes

All are examples that are helpful

reduce the force acting between the 2 surfaces

reduce the roughness of the 2 surfaces coming in contact

use wax, grease or oil

all of the above are ways of reducing friction

making the surfaces smoother

using lubricants such as waxes, oils, or grease

changing from sliding friction to rolling friction using ball bearings

using kitty litter on snow and ice to help your car gain traction