You folks may already know that I'm a huge advocate of harnesses for walking dogs, especially strong-pullers. Neck collars are okay for hanging ID and rabies tags, and serviceable enough if you're just walking a well-trained dog to your front yard to pee. But for strolls to the dog park or local pet store it's advisable to have something that's less stressful on your dog's neck.
I've walked dogs that could win an Iditarod. Yeah, Tucker, I'm talkin' to you! I told his parents that if their car ever broke down they could easily harness him up and yell "Mush!" Tucker almost dragged me into Shannon Lake a couple of times. Luckily, I'm used to dealing with strong dogs. More importantly, Tucker was harnessed.
Besides adding extra protection for dealing with strong dogs, using a harness can keep your dog from incurring a horrible set of injuries and maladies from the pressure on their neck. Check out this graphic for a good rundown of the problems that can result.
You can find good harnesses at all of the local pet suppliers. Myself, I'm an evangelist for Larz Pet Gear, which is only available through mail-order. But they're so worth it! I carry a Larz leash with me everywhere I go (it's the ONLY leash that I used to walk Tucker), and my big kids Snoopy and Holly have their harness. I don't get any kind of commission or anything, but I highly recommend them.
But even a Dollar Store harness is better than a neck collar if you have a dog that pulls. Just make sure you choose a harness that fits snug enough to work but not too tight around the chest. And you want something they can wear comfortably in all kinds of weather. A harness with heavy leather side molding can be torture on a furry dog in the summer.