http://www.positive-parents.org/2011/07/positive-parenting-in-action-aggressive.html
 You can print this and hang it up if you'd like, because these are the basic steps we'll follow in handling aggression.
Let's get right into the scenarios.
Scenario #1
Your  3 year old has become aggressive toward her baby sister. She tries to  hit her and push her over. You're concerned she's really going to hurt  the baby.
Reason behind the behavior: Jealousy, probably. It's hard sharing mom and dad, especially when you used to have them all to yourself.
:ACTION: Follow the above list.
1. Set a limit. (“We don’t hit”)
2. Offer empathy and acceptance of her feelings. (“You are disappointed”)
3. Let her discharge her feelings by crying with your comfort.
4. Help her explore ways to shift her mood.
To  expand on this a bit, you will take her safely away from the baby, get  down eye-level with her, and set the limit - we don't hit (or push, or  bite). It is important to acknowledge her feelings of anger or  frustration or jealousy that caused her to hit. "You're feeling upset at  the baby. Are you upset that I was holding her?" or "She grabbed your  toy and that made you angry." Your child is hurting, even though she may  look like she isn't. She needs to know it's safe to show her feelings.  Tell her it's okay to be angry, and its okay to cry, and that you will  keep everyone safe. If she melts down in your arms, she is healing. Let  her get her emotions out while you provide comfort. After the incident  is over and everyone is calm, address the reason behind the behavior.
1.  Spend special one-on-one time with each child. Let her pick the  activity. Connect with her. She needs to know that she is still just as  loved as before.
2. Teach appropriate ways to handle anger. You  can do this by talking it through, modeling it, role-playing, puppet  shows, books, or stories.
3. Don't punish her for hitting. At 3,  remember she didn't have the cognitive resources to stop and think  about her actions logically. 
"Punishment is not actually an  enforcement of the limits. That's our rationalization for punishing,  because we're frustrated that he isn't respecting our limits. Punishment  is actually retaliation, and retaliation always sabotages your  relationship with your child (or anyone else.)" - Dr. Laura Markham.
Teaching her how to handle her anger will serve her much better than punishing her for handling it wrong.
4. Read books to her about the baby and about being a big sister. For a list of such books, 
click here.
Scenario #2Your 19-month-old is a biter. He has just bitten another child at a play date.
Behind the behavior: It depends on what was happening at the play date. It could be frustration, anger, hurt feelings, or fear.
:ACTION:  Remember the steps above. Remove your little biter to safety, make sure  the child bitten is okay, and then set or reinforce your limit. "We  don't bite." Validate his feelings, empathize with his upset. "You got  mad because he took your truck. I see you're mad, but we don't bite.  Biting hurts." Let your child express his emotion safely, and  problem-solve later. The reason I suggest not talking about appropriate  alternatives during the time it happens is because children do not take  information in well when they are in "fight or flight" mode or are  upset. They are much more likely to learn and retain information when  they are calm. For more on toddler biting, read 
this article at TEACH Through Love.
Don't  bite him to show him how it feels. You'd be surprised at how many  parents would advise you to do this. Remember, you are the model for  appropriate behavior!
Scenario #3
You got a call from school. Your 8-year-old son punched another student for calling him a bad name.
Reason behind the behavior: Anger, obviously. Lack of ability to control his actions.
:ACTION:  We're not dealing with a toddler or preschooler now. An 8 year old  should have access to those higher brain functions. In other words, he 
should have been  able to pause and think about his actions. This is sometimes hard for  adults to do, however, so it isn't surprising that a child hasn't  mastered this yet. When you pick him up from school, you're going to  have to control your own anger. Model! Reserve judgment and ask him what  happened. Empathize with his hurt feelings at being called a name. It  does hurt! Now, because this is an older child, you may be tempted to  punish or give him a consequence, but that isn't going to solve the  problem or teach him how to handle a situation like this better the next  time. It's time to problem-solve. Remember the 
problem-solving post? Let him do most of the problem-solving with your guidance as needed. You might ask:
1.  How can you fix what you've done, because the student you punched is  hurt too? If he doesn't come up with an answer, offer a few  alternatives, such as call and apologize or write an apology letter.
2.  What can you do the next time you get called a name or there is a  confrontation? Let him brainstorm. It's good if he comes up with  alternatives on his own. If he draws a blank, help him out. You may  suggest he walk away, work it out with words, get help from an adult if  the situation requires it.
SUMMARYI'd like  to leave you with one more wonderful piece of advice from Laura Markham,  Ph.D.  She left this response on PPTB for a mother whose 3 year old was  acting aggressively, and it is a wonderful connecting game to play with  children to get rid of those nasty feelings underneath that cause  aggression.
Children who act aggressively are always acting  out of fear. Your 3 year old is afraid. Maybe she's afraid that he's  loved more? In that case, I would address that fear directly and try to  heal it.
For instance, play this game with her every single day  for the next week, to let her giggle off her fear and convince her you  adore her. Every day, spend 20 minutes playing the bumbler as you chase  her, hug, kiss, let her get away and repeat again and again: "I need my 3  year old fix....You can't get away...I have to hug you and cover you  with kisses....oh, no, you got away...I'm coming after you....I just  have to kiss you more and hug you more....You're too fast for me....But  I'll never give up...I love you too much...I got you....Now I'll kiss  your toes....Oh, no, you're too strong for me...But I will always want  more 3 year old hugs...."
This kind of game accomplishes at least 3 wonderful things:
1.  Giggling discharges the same stress hormones as crying or tantrumming  and thus makes kids happier and less stressed, thus less likely to "act  out" aggressively.
2. Kids are less aggressive and more cooperative when they have a daily chance to vent.
3.  This game also deepens your relationship with your daughter and  convinces her on a deep level that she is truly loved, dissolving her  fears and allowing her to be generous to her brother.
That  generosity is what makes your daughter care about the natural  consequence of hurting her brother, and gives her the competing impulse  of empathy to control her aggression.
Make sense? There are, of  course, many ways to address your three year old's big feelings, but I  love this game. I have never seen a child who did not respond to it. --  Laura
Aggressive behavior is very common in young children,  and peaks from ages 2-6. While this is a common phase kids go through,  it is our responsibility to set appropriate limits and teach  alternatives. Discipline is always about teaching them right, not  punishing the wrong. With empathy and loving guidance, your child will  learn appropriate ways to handle her emotions, and this phase will  become a distant memory.