Travel 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.)