Try the tennis ball technique. Hold as many tennis balls you can and have your dog on his show lead. Go somewhere with little distractions with a lot of room (like a field or empty dog park). Start gaiting your dog as you normally would, then after several strides, roll a tennis ball as far as you can, and give him the command, "Go!" or whatever you wish to use. Keep heading towards it at a trot, DO NOT run. If your dog tries to run, just correct him and keep him trotting. Then, before you get to the tennis ball you've just thrown (so your dog won't try to pick it up), roll the next tennis ball. This technique teaches the dog to go forward. It's also good for dogs who want to look up at you while gaiting. It has worked wonders with many show dogs. If your dog ignores the tennis balls and is not toy driven, try correcting the crabbing with the lead. When he starts to crab, tug the leash and say no. You may also be holding your lead too tight, too loose, or you may be going too slow, or too fast. Figure out what's right for your dog.