HI BArneydog

Here's how i teach the Leave It; I will try to keep it concise so if you don't understand anything just ask again!
I start with a food treat - something nice and smelly such as liver, chicken etc. It must be something he loves.
Sit on a chair with dog on lead. Hold treat in fist, tightly closed.
He will try to get the treat, will push at your hand etc, but will eventually sit back and look at you. (Ignore any other behaviour such as barking, or whatever. Wait until he "gives up")
When he does, open hand and let him have it (the treat that is! :D )
Make sure he removes his nose each time before you allow him the treat. Keep hand still, so he has to effectiveley move towards you to get the treat from your open hand.
Next - introduce the words; Leave It or whatever, and "Take it, Yours, OK" etc for when he is allowed the food.
Next, practice with an open hand every time...make sure he does not get the food if he does try to snatch, but he should be getting the idea by this stage.
He starts to realise Leave It means he never ever gets what he is after....but it is OK that he has the treat later, as it is on your say-so.
Then progress to treat on the floor, dog still on lead, same scenario. Your hand or foot should be ready to cover up the treat ..... in case!!!
It is so great when the dog just sits back and looks at you, knowing that is the only way to get the reward.
i use this as a basic exercise to start the dog off with the idea of what is required from him.
To generalise - VERY important - do this out on walks too, else he will only obey in your living room or wherever LOL!!

Next, on a walk, practice letting him sniff boring items; when he is nearly ready to move on, ask him to Leave It and when he does treat him and praise. (I use a clicker but it's OK if you dont want to or don't know how).
Next, progress out and about (but do it on a relaxed lead) to Leaving other dogs. At first make good use of your dog's critical distance and start well away fr0m other dogs, gradually over weeks get closer. Maybe get dog owning friends to help!
This is where knowing your dog's ultimate reward also comes in handy: by this stage I personally have progressed to Kong on a rope thrown as a reward as this is my girl's favourite game.
Meanwhile, to really sharpen up all this, get 2 toys. One he thinks is OK, one he adores utterly. Have him on a lead (I prefer to use a harness here as he won't get jerked) and throw his least favourite toy. AS he runs, shout "Leave It" and, if you have done the groundwork, he will immediately return to you for his best toy.
If at all possible dont let him get hold of the "OK" toy; the whole point is to teach a dog it ALWAYS fails when it hears those magic words "Leave It". A return to the handler may mean something nice though

Practice the last few exercises often, set your dog up to succeed and be confident about what you have on offer. It is important to not just rely on food here - by this stage, it is the relationship between you and your dog that counts. He must respect your control, know he will fail if he tries to go on after hearing the magic words, and be keen to return to you for praise and fun.
Done regularly this will give you great control and you can be confident.
My bitch who LOVES to chase put up a fallow deer a few days ago, just a few feet form her, and i was able to be calm, say "Leave it" in a normal voice, and she Left It - raced back to me at high speed for next instructions. I did of course give her a huge rerward of a Kong chase because she deserved it

and i was overjoyed!!!.Just wanted to say that so you know I am not talking out of my wotsit :D and it really does work in real life emergency situations.
I consider I am in fact still training her on all this thiough; but at the end of the day you are aiming for owner confidence, and also to know the dog may not necessarily get a reward every single time. I still like to surprise with rewards though because I love her face when she gets one!! I get something out of it too

I also feel in the case of such an important command, she will respond immediatley and not half heartedly so I tend to keep up the toys on this one.
If your dog does a half hearted Leave It at any stage, don't give a reward at all unless you are early on in the training. You have to gradually "up" the criteria so that only the very best response will do

Sorry this is sooo long,I did try to condense it, honest I did! HTH a bit anyway.
LIndsay