1-make the surfaces smoother. Rough surfaces provide more friction; smoother surfaces reduce friction.
2-lubrication. Lubricant is a slippery substance that reduces friction by making a surface smoother.
3-reduce contact between surfaces.
4-reduce forces acting on surfaces. Stronger forces means higher friction; reducing the forces reduces the friction.
5-make a more streamlined object. Streamline shape allows water or air to flow around it easily, offering the least resistance.