In The Sims 2 it was so easy to be immortal!  All you needed was a Sim who went far enough with the Biology career track to earn a Cow Plant, and a disposable Sim.  (Neighbors were handy for this.)  The Cow Plant lured in its victims with what looked like a slice of tasty cake, it ate them, and then you could milk it, drink the milk, and get days knocked off your lifespan.

Things are a lot tougher in Sims 3.  However, all is not lost, because you can make Ambrosia!  It's not easy, though.  I spent the better part of this weekend trying to get to the Ambrosia making point and failing.  Here is what I learned that you need:

A Life Fruit

The Life Fruit can only be grown from seed by a gardener (in other words, you can't just buy it from the grocery store or find it around town). 

You need to have level 7 Gardening skill in order to grow and harvest the Life Fruit.  It looks like a golden pear wearing a halo.

The Life Fruit is an Unknown Special Seed, so this is what you will want to look for when you go seed hunting.  The Collection Helper is invaluable for this project!  

A Death Fish

There are two ways that you can catch a Death Fish: you can be a Level 7 Angler who has read the $2,000 book "Fishing for the Dead: Death fish and Vampire fish on the Line."  Or you can be a Level 10 Angler.

You can catch the fish at Level 7 by using an Angel Fish as bait, or at Level 10 with just the hook.  Your best bet for catching a Death Fish is to fish at the graveyard (or in Riverview, directly behind the graveyard, at the "no fishing" sign) between 11PM and 5AM.

A Cook

The recipe book for Ambrosia costs $12,000 and you need to have level 10 Cooking skill to cook it.  Sheesh!  

Strategy

Obviously the best strategy is to train three Sims in concert: a cook, a fisher, and a gardener.  If you plan far enough ahead in their lives (or use the Mid Life Crisis lifetime happiness reward) you can give Natural Chef trait to the cook, the Angler trait to the fisher, and the Green Thumb trait to the gardener.  I also recommend using an accelerator like the Genius trait, the "learn faster" happiness reward, or just making it a habit to study at the library.

The gardener will probably take the most time, if only because it takes many days between planting a Life Fruit seed and harvesting a Life Fruit.  You will definitely want to fertilize the seed with the best fertilizer you have on hand, because fertilizing a seed helps them to grow faster.  You can check this handy chart to cross-reference the ingredients you have on hand.  

If you want to make this a multi-generational challenge, it does seem that fruits and fish do not go bad in the refrigerator.  Imagine three generations of Sims  - the first generation grows the Life Fruit, the second generation catches Death Fish, and the third generation cooks Ambrosia!