2 comments before I offer another option to corrections: 1. Labs are pain insensitive - most gleefully put up with corrections 2. If you choose collar correction, you should see immediate results...If you do not, its because your corrections aren't severe enough (most of us aren't willing to be as severe as we have to be in order to get the message through - especially with pain insensitive dogs). Poorly enforced corrections are ineffective - they are meaningless to the dog and become a nuissance - which makes them less likely to notice an even harsher correction... Not to mention it never cures the pulling problem.
Training tools in general can a huge help but most people become dependent on them because its easy... It is actually harder to use training tools because it takes additional time and training to transition OFF of them. Many people never make that transition which of course means that NO training has taken place. So if you use them as a tool, great but don't let it replace training.
Increase physical and mental activity - it can be difficult to increase physical activity if you can't walk anywhere without pulling but it is necessary... Dogs who are understimulated and undersocialized are the worst pullers IMO so you really need to provide an outlet for your dog so he is not pulling out of frustration/boredom... At first you may have to play fetch in the backyard or make a flirt/lure pole or even drive to an off-leash area at off-peak hours so you don't reward any pulling. If you have the vet's okay - get your dog into weight pulling - it is wonderful outlet for all his energy and an acceptable time for pulling (you can also do this in your yard!). My dog used to pull a truck tire as conditioning exercises - it is definately exhausting (but you need to work up to that kind of weight!)
Practice loose leash walking indoors or in the backyard - anywhere that is relatively boring. If that's easy for him - add distractions like toys, food and enlist some volunteers (ie: neighbours and their dogs) so that you have increasingly difficult distractions to practice on before you get to the hardest distractions of all (walks).
When you do go on walks - go at less stimulating times and get to a quiet area fast. You may need to use a training tool but don't let it replace training. ONLY use it when you have no choice but go for a walk and once your dog is tired or if you are at a "boring" place (ie: very few distractions) take off the training tool and start training!!!!
On top of all this, you need to do some OB training - if your dog already knows the basics, then get started on some other useful behaviours or tricks... For example - "touch" - if you teach your dog to target your hand or a target stick (ie: touch it with his nose) you have a way or getting his focus back onto you and away from distractions. Work on self-control exercises like stays and leave its...
Now - all of this requires that you have a reward to give the dog. You mention that you used treats... What kind? Were they the best of the best? How did you deliver the treat?
Generally treats are the best things to use because they are easy to deliver - which is why so many people limit themselves to them... In fact, a REWARD is ANYTHING YOUR DOG WILL WORK FOR. If your dog has a special toy, use that instead (requires a good leave it) - I knew a dog who's best reward was pinecones. My dog loves water - he loves to catch splashes so I fill a water bottle and squirt water for him to catch as a reward. The only limits are your imagination and of course what your dog considers a reward. Some dogs will work for petting or praise - most need a little extra.
Another reward that people tend to overlook is FREEDOM... For example, if you are practicing loose leash walking in your backyard and your has just don't really well, releasing him to go sniff or just to run around with the leash dragging is a perfect reward...
One more thing, reward him anytime their is slack in the leash or even if there is no leash involved, reward him for being within the 4-6ft that you want him to be in on a walk... Dogs will quickly learn that the 6ft bubble around you is rewarding and will CHOOSE to come into it more often... Dogs who are given the opportunity to make the right choice are learning...whereas dogs who pull and are then corrected are still practicing the bad behaviour and if the corrections aren't enough of a detterent - they aren't learning at all.
"Animals are reliable, many full of love, true in their affections, predictable in their actions, grateful and loyal. Difficult standards for people to live up to."