Lead training is a long process because the reward the dog is pulling to is the best thing in their little world! suffering the pain of a gentle leader or any other ''stop pulling' harness which restricts the dogs movement and applies pressure is irrlevant to the dog.
In addition using the food reward method you may also want to add a clicker. this little plastic box with a tongue of metal makes a more consistant definitive sound that will help your dog to learn faster as all training is down to consistency.
To ween the dog off food or to make rewards higher value to him you need to randomise their quality. there's no reason other than weight gain why the dog couldnt have his whole dinner for a few steps of perfect lead work, he should however never receive nothing.
would you do a days work for free?
would you do another days work if you knoew you were going to get a fiver at the end
How many days would you ask the boss for if you got £1000 bonus occasionally for no more effort?
Lead walking will take longer to teach if you're still walking the dog in 'the real world' I'd buy a non punitive harness such as the fleece padded ones from
www.spiffydog.co.uk a small spaniel is a size 3 a larger spaniel is a 4. (lab is 4 to 5) dogs are rubbish at generalising so you will need to practise what you've accomplished in your living room, back garden and hall way out in the street probably from scratch or atleast a few steps back.
When you're out training have the dog on a collar if that's your preference
when you're out for a walk without training a halti head collar (not a gentle leader or gencon!) or harness will give the dog a clear signal this is not training, there is however no reason why if the dog offers you the behaviour without lure or prompt why he/she should not receive a treat/or other reward.
If you're a complete purist you'll want to wait until the dog offers the behaviour before adding a command to it.
When luring your dog into the heel position you'll want to do 3 repeats with food, 1 without in your hand, then 3 repeats with food and 2 without..etc to wean off the food in hand lure (or you'll end up hooked on it)
I hope that tacks onto the above post on heal training and provides some tips
NB - I'm against the gencon and gentle leader due to the fact they press HARD on the dog's pressure points on the head regardless of whether the dog is walking relaxed or towing. A halti if used properly will work. There is still no substitute for modern science based training over any tool you can buy.