a) Never coerce the "come."
b) Never punish or chastise an imperfect response.
c) Never call a dog to punish him.
And remember, in order to solidify the response during every day life use the "come" command plus a click and treat for coming when you have anything special to offer to your dog e.g. a car ride, dinner, a special food treat, or new toy.
Walking to Heel Actually, walking to heel is not very important, but walking with a slack lead and not pulling is important. As usual with clicker training, start with baby steps. Attach the lead and coax your dog to stand at your left side by patting your left thigh. Click, reward. Take a pace forward and coax him to join you – not with a food treat as a lure - by calling him along enthusiastically. "Come on Buddy, let's go" (patting thigh). If (and when) he takes a pace forward click-treat. Click for one pace, then for two, and so on. Pretty soon you'll be up and walking. Click him while he is in motion. That stops him; then give the treat and off you go again.
Voice Commands We mentioned using voice cues to train come, but not sit, down, or walk to heel. Don't worry, commands can be added later, after the behavior has been shaped. Your dog will soon learn that he is rewarded if he responds in the desired way only after the requisite command has been given.