I responded to a similar post before so I have copied and pasted that here
" My son is 15 months and has also started his "terrible twos" a bit early [actually he started at 13 months]. He hits, bites, screams, scratches, and has recently discovered he can kick while I change his diaper. I think it is normal, and it sounds like you are doing fine. I would suggest continuing the stern "no hitting!" and tell him, "that hurts mommy" or "i don't like it when you hit me" or something like that. When you put him on a time out, make sure he cannot see you. The isolation is terrible for any child but that is why time-outs are so effective. The distraction, the little bit of time to cool off and break the bad mood. Also, what I have done with my son is show him how to "be gentle" by taking his hand and stroking your arm. My son also hits the cat, and after doing this, now I just have to say "be gentle" and he will pet him nicely. I also try to think of the root cause of his behavior. I go through the list in my mind.... Is he tired? did he not sleep well last night? starting to get hungry? did the fruit at lunch give him a sugar rush? Have I been on the phone or at the computer a bit too much and he needs some more attention? how have I stimulated his mind so far today? have we been outside yet today? has he had a chance to run around and burn off some energy? Or is this purely a toddler power struggle? Sometimes he gets into a negative mood and continues to misbehave until I figure out what he really needs [since he can't tell me yet]
Most of the time, however, I think he is just testing the waters to see how I will react, or to see how far he can push it before I lose my patience.
Another thing I have learned is to pick my battles and try to understand where he is coming from, and his mental ability and how he communicates. Sometimes I get really frustrated, but then I think to myself...okay, he basically only points and screams to communicate his needs/desires, so at this point in his life, how else is he supposed to communicate his anger or frustration?
-I just wanted to add: Another thing I realized is that as time went on, I started slacking with baby-proofing. He started walking at 8 months and at that point everything was out of reach. But I tend to let the dining room table get cluttered with mail, my bags, books, ...basically that is my place to set things down. When my son was tall enough to reach up there, I found myself taking things away from him all day long. It was my fault that I was getting so frustrated with him. I made an excuse for myself: well he understands "no" and should be listening to me when I say it to him! ....But of course he will want to explore everything he can get his hands on. He is learning every day and my bags and books are especially interesting because I wont let him have those things! So I've done my best to re-baby proof the room for a taller toddler and that has made things a lot easier "
This also relates to your situation:
" If she pees, make her help clean it up. My 16 month old son has had lots of fun making messes with his bottles/sippy cups [tipping it over and watching the formula drip out, pushing the nipple down into the bottle and then pouring the liquid out, unscrewing the top and pouring it on the floor, and then spreading it around with his hands] and about a month ago, I started to make him help me wipe it up. I would tell him "no, we don't______" "you made a mess, let's clean it up."
I gave him a wipe, and held his hand and wiped it up with him. I kept doing this until he got the idea, and then I started to tell him "you made a mess" and to "wipe the mess" "clean it up" without my help. Now, only a month later, he is very careful about not tipping his bottle or sippy over. He even puts it up on the table by himself when he is ready to move onto something else. Actually, this approach has nearly eliminated his throwing food off of the highchair as well. Obviously when he cleans something he doesn't do it very well and I still have to wipe up what he missed, but I don't mind since he is helping and learning a lesson. When he throws Cheerios off of the highchair however, I don't help other than if he misses some, and I'll tell him "honey there are more over here, please pick them up" "thank you". "
I have used that technique for my son's aggressive behavior too [Basically the Super Nanny technique]. When he hits me I tell him "No. We don't hit. You gave mommy an owie" and immediately put him on the time out chair. I tell him what he did wrong and why it's bad and make him sit there for about a minute and at the end I repeat what he did and why it is bad, and tell him to give me a hug. It has been working wonders for us. He hardly ever hits anymore. You said he hits his own head on the coffee table--I would use this same technique for that also. If he hurts himself, put him on the time-out and say "No, we don't hit ourselves. That hurts and makes Nixon and mommy very sad." Make him sit there as usual and explain it again, and then have him give himself a hug.
I think a consequence, combined with "making amends" for the wrong-doing is very effective because it helps them learn empathy as well as accountability.
I don't want to make you worry, but another thing that I thought of when reading your post initially was that symptoms of Autism sometimes start to show around 18 months, and while tantrums are perfectly normal for every child, sometimes extreme meltdowns that include violence and self-injury could be one of the symptoms of Autism or other disorders. Here are a couple of sites that talk about it. If you have seen other signs that something is "not quite right" it might be worth it to bring it up to your son's Doctor and see what they think
http://childbrain.com/pddq5.shtml
http://www.brighttots.com/autism_and_tantrums
The second site has great information and here is part of that page:
Difficult behavior that frequently lasts longer than 15 minutes, occurs more than 3 times a day, or is more aggressive may indicate that a child has an underlying medical, emotional, or social problem that needs attention. These are not considered typical temper tantrums. Difficult behaviors may include:
•Kicking, hitting, biting, scratching, hair pulling, or pinching other people.
•Throwing or breaking things.
•Head-banging or inflicting self-injury.
•The child's temper tantrums frequently last longer than 15 minutes or occur more than 3 times a day.
•The child's behavior does not improve after 4 years of age.
•The child hurts him/her self, other people, or objects during a temper tantrum.
•You have problems handling your child's behavior, especially if you are concerned that you might hurt your child.
•You want help with learning to cope with your feelings during your child's temper tantrums