When your toddler begins his willful stage, it's helpful to know there is a way you can manage his behavior and make clear your expectations.

Here are a few simple steps to get started.
Grab and run: This is your toddler's favorite new game, much to the chagrin of his now-wailing playmates and his much-embarrassed parent (that'd be you). With his hands full of whatever his little buddy was playing with and his face plastered with a mischievous smirk, your toddler is a prime candidate for one of the most popular discipline tactics out there: the time-out. The idea behind an effective time-out is that you are taking away the most important thing to your toddler—your parental attention.
Time-Outs, Step by Step
First, briefly point out the offending behavior and announce that your child is in time-out: "We don't hit. You are in time out." Keep it short and simple. Then, lead your child to a spot where he can sit that is safe and totally non-entertaining. The bottom stair for older kids, a special chair, a quiet corner, or a "time-out cushion" work well. Situate your toddler and set a timer. An egg timer, a digital timer, and the timer on your microwave all work well. One minute of time-out per year of age is the standard recommendation. You might need to start with a much shorter time frame, such as a flat 20 seconds, when you first begin. And voila! You have introduced the concept of time-out!
Time-Out Considerations
Keep in mind these tips when disciplining:
Any effort to regain your attention during this time should be ignored. If your child gets up or requires your attention, the timer should be reset.
At the end of the time-out, comfort your child and briefly explain your future expectations: "We don't hit when we don't get our way. Next time tell Mommy what you need."
Stay calm. Few parents think rationally when mad. And a toddler knows how to push just the right buttons!
Be consistent. If hitting our baby sister is sometimes ignored (or giggled at) and sometimes punished, what message are we sending? On the other hand, using a time-out for behaviors which occur frequently diminishes its effectiveness.
Positive reinforcement is the best behavioral modifier. Catch your child during those times he is being gentle with the baby and point it out.
And just like all "expert" advice, time-outs don't work for all parents or all kids. If your discipline technique is not working, talk to your pediatrician.