I have a gorgeous GS bitch who is nearly 5mths old.  The last couple of days she has begun barking at anything and everything, other dogs we meet, people on the other side of the road, cars, me! I am now very wary of letting her off the lead as she is straining to get to these other people/dogs.  Yesterday something awful happened she saw 2 small kids with their mother and even though she is doing well with training, certainly coming back when called, she was off after these kids barking like mad and scared them half to death.  I relise she is young yet, but I am anxious to put a stop to any naughty behaviour before it really gets a hold. Any ideas please.

Set up a situation where your dog is on a long line (so it cannot attack anything) with you behind it and will bark at a distraction. As soon as the dog starts barking throw a bunch of keys, a spare check chain, small shaker tin or similar very hard at it's rear end. You have to be unequivocal with this correction so do it as hard as you can. It is vital that as soon as the dog turns towards you you call it to you in a friendly voice and give it lots of love and reassurance that you are still friendly. I have used this method on ex-Rescue working/guard dogs and it does work. If you are unsure about your aim practice somewhere where the dog cannot see or hear you. 

You shouldn't throw the keys or whatever AT the dog but behind it(the late John Fishers training discs have advice about use)

You can now buy these discs under the MIKKI brand, they come with an instruction booklet which is easy to understand and put into practice :)

Well said Moonmaiden this kind of aversion therapy is meant to interrupt the behaviour by the sound of the keys, discs whatever hitting the ground NOT by means of pain on being hit by flying missiles!