There are a lot of changes to the way Sims can move out in Sims 3.  Back in Sims 2, you just made a phone call, then walked off into the ethereal blue mist.  Or you could evict the entire family, and move them into a new house.  You could invite other family members to move in with you, but the whole thing was very clunky.  I groaned whenever I had to do anything involving the Family Bin, because somehow I always screwed it up.

In The Sims 3, all you have to do is place a call (or click on the computer, or use the newspaper).  (And who are you calling, anyway?  They never do explain that part…)  You then get a dialogue box which gives you your existing Sims household on one side, and the new household on the other side.  This is the same as the marriage dialogue, and accomplishes basically the same goals.

If you want one Sim to move out, then shift them over to the right hand window.  If you want to stay with your other Sims, you can tick the box to "kick out" everyone on the right hand side.  This makes them leave the household, while keeping your playable household intact.

(One fun thing about choosing to kick a Sim out of the house is that it isn't as permanent as it sounds.  Your kicked out Sim will find their own place, and keep living their life.  I have run into the descendents of Sims that I kicked out - most of them are doing surprisingly well for themselves!  They have married, and had kids, and moved into pretty nice houses.  They almost seem to do better than the Sims that I control myself.  Hmm.)

Before you moved out in The Sims 2, you always wanted to be sure to stock your inventory with as much loot as you could find.  Paintings and statues were great for this.  Expensive high end items could then be sold once you moved into your new house.  An excellent way to take some of the family's wealth with you!

This strategy isn't necessary in The Sims 3, because the Sims who are moving out basically take all the house's money with them.  This leaves your left behind Sims with nothing.  Try not to think about that.

Before you move, you will want to "pack."  First, I come through every Sim's inventory, and put things back where they belong.  Fish and produce go into the fridge, books go into the bookcase, and so forth.  Small items I want to take with me, like cameras, guitars, laptops, and the Collection Helper, I drag out onto the floor.  And of course if you have the deeds to a store, click to transfer that deed to your Sim who is leaving.

Next, I go through and take everything out of the fridge, the bookcases, and the nectar racks.  It took a long time to accumulate that stuff, and I don't want to leave it behind!  I have built up a book collection that is big enough to require a second bookcase.  

Fortunately, the game makes this easy for you.  Just click to "Open Item" ("Open Bookcase," "Open Fridge," etc) then click the "Take All" button.  Then drag all those little things off the floor and into your backpack, and you're ready to go!