Mars has excellent conditions for mummification.
There isn't much you actually have to do except leave a body on the surface. Mars will do the rest. The corpse will desiccate fairly quickly and will then hardly decompose over the centuries. With the very cold climate, minimal and dry atmosphere, and no microbial life there isn't much that will be able to decompose it.
The main issue will be wind erosion, but it's minor, and mummies have been found in less ideal environments. An example of these is seen in Antarctica, some seal mummies have been found on the surface that are as much as 2,600 years old. It's worth noting these are accidental mummifications, the animals simply died there and mummified naturally. They still have skin after all that time, the bones will last much longer.
As mentioned by Jake, the Atacama desert is another excellent example as it's used by NASA to simulate a Martian environment. Some of the mummies found there are almost 9,000 years old. Granted, those mummies were intentionally prepared, but clearly the environment is highly favorable for mummies.
Mars is better.
Earth mummies have active microbial life chipping away at them, Mars won't have that. The only microbial life will be that which was brought in the gut of the human, and it is not suited for survival in those conditions. The gut bacteria will be the last thing in the human to die, but it will die.
Aside from the small amount of wind, it's pretty similar to being a corpse in space. Finding a 50,000 year old skeleton or mummy on Mars would be shocking indeed, but not because we would have expected it to decay. If a human died on Mars in the distant past, it would actually be a surprise not to find the body.