This is a very common problem, but very easy to fix.
Here are some fun, positive ways to teach a young puppy to keep a loose leash...
Have your pup sitting or standing at your side. Then, let him know to begin walking by saying "Let's go!" and begin walking. Praise your puppy for staying at your side. If she does bolt ahead, freeze. Act like a stick in the mud. Once she's in a calm state again, call her back to your side and start walking again. Keep repeating this until she finally understands that by staying at your side, she will be able to move foward.
Start off with your puppy at your side at a "start line". Have something your puppy really wants (your goal) about 20ft. away. This "goal" can be a treat, toy, person, or another dog. Let your dog know to begin walking by saying "Let's go!" and begin walking. If your puppy pulls, don't yank the leash or scream and yell. Just gently bring your dog back to the start line. Once she's calm, begin walking once again. Keep repeating this until you can finally make it all the way to the goal without your pup pulling. When she reaches the goal, praise her, treat her, and give her the "goal".
Start with your puppy at your side. Take three steps and treat your dog for not pulling. Do this repeatedly and gradually increase the distance you walk until your puppy is walking with a loose leash.
You can also play the "Catch Me" game. Although I wouldn't consider it a method for training loose leash walking, it will teach your puppy that being near you is a good thing.
In a secure area, let your puppy loose and walk briskly, jog, or run. Don't acknowledge your puppy or call her to you, just act like you're having loads of fun running around. When she runs to your left side praise her and give her lots of treats. Let her know that staying at your left side is THE BEST place to be. Keep running, walking, or jogging, and continue to praise and treat your puppy for coming to your left side. If she slows down and doesn't come to your left side, speed yourself up a bit.
I hope that helped! (;