I have 2 kids girl(15) who basically did as she was told & I thought well I have seen where I thought friends & family were getting it wrong, learnt & i'm doing it "right" then 6 & 1/2 years later I had my son & discovered how wrong I was (& still am - he's

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Nevertheless I do believe that being consistent is really important, kids need to know where they stand & why, don't just say "because I say so" explain, tell them why they can't do something, don't say you'll take them to the shops/park whatever & not do it, keep promises or say up front that you'll take them if the weather is fine, or if theybehave or whatever, let them know where they stand.
Next, if you say it do it, whatever it is, from a fast food meal to a punishment - stick to your guns. (i really like the advice about shutting the child in their room (take out all toys if need be) then turn on music loud!! & do something to occupy yourself till the screaming/throwing etc stops).
If they start doing all this when you're out, the best thing is to just pack them up screams & all & go home. I know it's awful when you're out & everyone stares & comments but the sooner you pack up & go the sooner you're out from public (or family) comment & you can just deal with what you have to deal with.
With the hitting, (now I do happen to think that within reason smacking is ok, but not always, each child/situation is different) & the head banging (which could cause long term damage depending what they hit & how often etc) i'd suggest something that's great if they try holding their breath - pour a glass of cold water over them - the shock makes them breathe & it may also shock themn out of the head banging/hitting. Another possible way to handle the hitting especially, is to sit the child down (at a time when they'er not doing this!!) & say if you hit me I will take "then name a favourite toy" off you for a day, if you hit me again you'll lose it for a week & so on. You may have to actually reach a poiint where you give it away or throw it out so choose a loved item but not an expensive one.
All this said & I still do it all & alot more but my son is a totally "wild thing" he runs off/takes off/goes out without permission no matter what I do & I think i've done it all, still any & all advice welcome (i have thought of getting a microchip put in - but I don't think they do it for kids just pets!!

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good luck to us all. I think a parents main role is to survive being a parent!!