Near the gate at Pleasant Rest Graveyard there is a bush called 'Death Flower Bush'. Harvest the flower from it and keep it in your Sim's inventory and if you die that Sim will give that flower to the Grim Reaper and be resurrected. The flower will not reappear there again but 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 grow as a Death Flower. It is also worth having the Green Thumb trait which can be used to revive the plant.