Go to Pleasant Rest Graveyard, and near the gate, there is a strange bush called a 
"Death Flower Bush" - Harvest the flower from it. Keep this flower in your sim's 
inventory, and if you die, you will automatically offer this flower to the Grim Reaper,
who will ressurect you in exchange for it. This flower will not reappear, but there are
Unknown Special Seeds (requires level 7 Gardening to plant) dotted about near the 
graveyard (and a few other places) which may grow another Death Flower (and sometimes
more if your Sim has the Green Thumb trait to revive the plant.)