First things first: Is today a good day to start? If you're in the midst of potty training, are going on vacation, or are expecting a new baby, wait until things settle down. Yes, you'll have to put up with your little bed partner longer, but the sleep training will go much faster if you wait until your routine is more regular.
Once you've decided to take the plunge, start talking about your new bedtime expectations in the afternoon -- that way, she'll know what to expect at lights-out. Try saying something like "Mommies and daddies sleep in their beds, and kids sleep in their own beds," says Spivack.
She also suggests making a homemade "sleepytime book" -- nothing fancy, just stapled-together paper illustrated with stick-figure pictures that your child can color. If your family recently moved, for instance, and your daughter started sleeping in your bed while she got used to the new house, your story would focus on that and end with how she finally started sleeping happily in her very own bed. A picture book can help young children understand their new sleeping situation in a very concrete way.
Do your usual bedtime routine (here's help if you don't have one!), then get ready for the boot-camp -- tough part.