Walking on a loose leash

The call I get the most as a dog trainer is from frustrated owner's who are tired of their dog dragging them down the street when going for a walk.   Because the walk is such an unpleasant experience, the owner takes the dog on less walks, which increases the dog's excitement when it does get to go, which in turn makes the walk more unpleasant. It becomes a vicious cycle and both the owner and the dog suffer as a result.  
There are a multitude of training collars, leashes, harnesses and devices out on the market today that are supposed to make walking your dog easier.   I won't get into the pros and cons of each one of these devices on here, otherwise this post would be a mile long.  Training your dog to walk on a loose leash, with just a regular flat buckle collar is possible.  It has a lot less to do with the tool, than it does with your technique.  
If you have a puppy, get them used to walking around with a collar and leash right away.  Whether your dog is a pup, or a grown adult, start inside your home where there are as few distractions as possible.  Start to teach them to move away from leash pressure.  Wait for there to be slack in the leash and reward the slack with praise and a yummy treat.  Then let your dog go off exploring again. Once the leash becomes taut, wait it out.  Reward each time the leash becomes slack. It won't take your dog long to figure it out.  A tight leash, gets no reward, whereas a slack leash gets yummy treats.  Keep in mind what is rewarding, just being able to move forward is a reward for your dog, if the leash it taut, moving forward can not happen.  Ever.  If they pull...stop.   Wait for the leash to be slack and reward.  Turn around, change directions.  Be interesting to your dog.  Praise them and give them treats when they are walking beside you.  
Once your dog has mastered moving away from leash pressure indoors, take them out to the back yard and start all over again.  At first they may regress as there are a lot more interesting things to see and smell outside than there was inside your house.  Keep up the practice.  Rewarding the dog for walking beside you with a slack leash and not rewarding the pulling by moving forward.  Do not move on to a more distracting environment until your dog has mastered this technique in your back yard.  It is very important that while you are doing this training with your dog that you don't ever allow it to pull you on a walk. You will need to get creative with other ways to exercise your dog that doesn't involve walks.  
Once your dog has mastered the release of leash pressure in your backyard, move to the front yard.  Once that is mastered, take the walk on the road.  Just ensure that you continue to praise, and treat when your dog is beside you, and do not allow your dog to pull you down the road at any point.  If they start to pull, turn around and go in the opposite direction.  Walk briskly, stay interesting to your dog.  Be unpredictable, keep them guessing what you are up to and which way you will be going next.   The amount of time this takes to perfect, will depend on your dog and your consistency.  If your dog has had years of dragging you down the street, it is still possible to change that habit into a calm, relaxed, engaging walk.   One in which both of you can enjoy.