These people are loaded.

Top-earning dead celebrities
Rank Name Earnings ($mil)
1 Elvis Presley 40
2 Charles Schulz 32
3 J.R.R. Tolkien 22
4 John Lennon 19
5 George Harrison 16
6 Theodor "Dr. Seuss" Geisel 16
7 Dale Earnhardt 15
8 Tupac Shakur 12
9 Bob Marley 9
10 Marilyn Monroe 8

Clearly the secret is to produce something of the artistic sort that will be in demand long after you're gone.