Assuming simple curiosity is driving your question, the answer depends on whom you ask.
The ancient Egyptians would say if you had enough wealth and status, special priests could preserve you. Forever.
First, they pull out your brain and internal organs, except your heart. The heart stays. Then they dry out your body with a kind of salt. Most of your organs would be preserved in jars, but your body would get wrapped up in strips of linen and layers of resin before being sealed in a coffin.
Their technique worked pretty well. But they weren't the first to go mummy. Two thousand years earlier, the Chinchorro of Chile and Peru were in the habit of skinning, dismembering and disemboweling bodies. After that they would putt them back together with sticks, straw and plant fibers. Then they'd paint the bodies.
More recently, nature did the dirty work for the Inca. They made sacrifices to their gods by bringing kids high up into the Andes Mountains. The kids' bodies became mummified by the dry atmosphere and extreme cold.
Peat bogs, glaciers and deserts can have the same effect. The oxygen-deprived acidic, cold, hot and dry conditions of these environments desiccate bodies and stop decomposition au natural.
So basically, take your pick. Wait, I mean, not really though, right?