With The Sims 3, every time your Sim fulfills a wish, they get Lifetime Happiness points.  These also accrue as your Sim's life passes along, and in fact your Sim can rack up a fair number of Lifetime Happiness points just by playing the game.

When you click on the "treasure chest" tab to view the Lifetime Happiness tallies, you can click to open a menu of items and rewards that you can buy.  There are an awful lot, and even more were added with the World Adventures expansion pack.  Clearly, you need a strategy!

The first thing you should do for each household is equip one Sim with the "No Bills Ever" reward.  This one is awesome, and can save you a ton of Simoleons!  (Only one Sim per household needs to have this reward, in order to free the entire household from bills.)

Next, consider whether or not you have a lifetime goal for your Sim.  If so, then there is probably a reward which can help you reach that goal.  

For example, the "Extra Creative" reward is handy if you have a painting-related goal.  "Stone Hearted" is great if you expect your Sim to have to ride out a lot of deaths or break-ups.  "Fertility Treatment" if you want to build a big family.  You get the idea.  

If you don't have a specific goal in mind, and someone in your household already has "No Bills Ever," then your next strategy is to pick rewards that affect your Sim's basic Motives.  There are six Motives: Hunger, Bladder, Energy, Social, Fun, and Hygeine.  If you read the descriptions carefully, there are rewards which make those Motives drop more slowly:

• Hunger: "Hardly Hungry"
• Bladder: "Steel Bladder"
• Energy: "Meditative Trance Sleep"
• Social: "Attractive" and "Never Dull"
• Fun: "Carefree"
• Hygeine: "Dirt Defiant"

Finally, there are four actual objects that you can buy with Lifetime Happiness points.  One nice thing about these is that you can pass them from one Sim to another.  This means that you really only need one per household.  Just drag the item out of your Sim's inventory onto the floor or countertop, then drag it into the other Sim's inventory.

Collection Helper
Every family should have one!  This puts a pushpin on the town map, showing the location of whatever item you select.  It finds Rocks and Gems, Fish, Seeds, and Insects.  When you click on the pushpin to zoom in, the item glows to make it easier to find.

Food Replicator
Mildly useful.  You can store great meals in it, and replicating a meal is faster than preparing it.  Children and teens can use it, which is handy.

Moodlet Manager
This lets you afflict a Sim with bad moodlets, or delete the bad moodlets of a Sim to make them happier.  It can be pretty handy, although it sometimes backfires.

Teleportation Pad
This is spiffy, but I never remember to use it.

Body Sculptor
In theory you can turn a Sim fat, thin, strong, or weak.  In practice it hardly ever works as well as you think it will, and it seems to backfire at least a third of the time.