Hi Carlo,
Most people condition a command such as "yes" or "find it" to indicate to the dog that it needs to switch on its nose and commence a tight hunt. The method I have been taught by a couple of successful triallers seems to do the job nicely.
Find yourself a nice bit of reasonably heavy cover (I just use long grass) and, with your dog at heel throw a neutral coloured dummy into the cover only a couple of metres. The principle is that the heavy cover will force the dog to hunt.
I send the dog in with the command "find it". This should be an easy retrieve. Next step for me is to put the dog away and toss the dummy in or to go for a walk and, when the dog isn't looking, toss the dummy into cover. Then, standing quite close to the dog I send the dog in with the command "find it". Big praise if it comes out with the dummy. If not, still enthusiastically send the dog back in with "find it".
When the dog is putting on a very keen hunt and coming up with the dummy every time without bailing out of the cover I make it a bit harder. I send the dog into cover without a dummy and command "find it" . Each time the dog ceases the hunt and bails out I send it back in with the command "find it". Then, without the dog seeing, I toss the dummy into the area so that the dog is rewarded for its efforts. The principle behind this is to teach the dog that,
if commanded to do so, it should persist with a hunt in a given area regardless of whether it believes there is anything there or not. It teaches the dog to trust your judgement.
All this initial work is conducted at close quarters. You want to stay close to ensure your dog maintains a tight hunt. If you throw the dummy into cover 30m away you will have little opportunity to encourage him to hunt the right area, he may not find the dummy, then you may not find the dummy, then he's learnt nothing and you'll have to find the phone number for Specialised Gundog Accessories to buy a new dummy
As I said, this method has been taught to me by some good Australian trainers. Its worked for me so far (in fact I've used it to encourage hunt in my work dogs as well!) and it appears to work for them.
Good luck!
Kirsty