Go to Pleasant Rest Graveyard. Near the gate is a bush called "Death Flower Bush". Harvest the flower from it and keep it in your Sim's inventory. If you die, that Sim will give that flower to the Grim Reaper and be resurrected. Although the flower will not reappear there again, if your Sim has level 7 Gardening they can plant the Unknown Special Seeds found in the graveyard and some other places that have a chance to grown as a Death Flower. Note: Having the Green Thumb trait to revive the plant may help.