In all seriousness, part one of my answer would be that your association (or any association in this situation) needs get an assignor with a spine. If an assignor takes away games when a coach calls and complains about an umpire, that's a big problem. An assignor should take away games solely based on the assignor's or the umpire association's evaluation of an umpire. With that said, I know that I don't have knowledge of the politics that goes on in certain parts of the country. Thus, I know that replacing a spineless assignor may be easier said than done. [I also know that there are some travel ball leagues/entities (I use term "league" loosely when speaking about travel ball) that care more about the coaches because the coaches pay money to play whereas umpires are viewed as nothing more than a necessary expense. My only advice to you on that is either to not work travel ball, find a travel ball entity that supports its umpires, or work the travel ball games with full knowledge that you have no support from the "higher ups". There is not much more I can say about "for-profit" travel ball entities.]
Part two of my answer is: even though you are not going to get help, you should still be approachable. For instance, if the coach comes and asks you to get help explain (in a calm tone of voice) why you are not going to get help. By way of example,
Coach, "Matt, he pulled his foot on that. I need you to get help."
Umpire, "Jim, I was in perfect position to see that play. I had a clear view of the fielder's foot touching the bag and I was standing 12-feet away. I do not need to get help from my partner on this call."
Coach, "You need to get help. Don't be bigger than the game."
Umpire, "Jim, if I had been blocked by a player, or if I had been out of position I assure you I would get help to get the call right. But I was in perfect position and had a perfect view of this play and I have this call right. Its time to get the game going."
I have not read "Verbal Judo", yet (it is on my to-do list). But, in my example you've addressed the coach's concerns with your calm responses. First, the coach was concerned that you got the call wrong (missed a pulled foot.) You have informed him that you had a clear, unobstructed view of the fielder's foot and thus don't need to get help. Then, when he argues that you're refusing to get help because either you're stubborn, have a big ego, or are "bigger than the game", you've addressed that by assuring him that you would get help if your view had been blocked by a player or if you were out of position. Finally, you've then taken control of the situation by saying that its now time to get the game going. If he keeps arguing...and you're working a FED game...use your three warnings at this point. Give him a verbal warning, "Jim, this is your verbal warning. I have clearly explained to you why I do not need to get help on this play. If you continue to argue you are going to be restricted to the dugout or ejected. It is time to get the game going." If he continues to argue, proceed to the next steps.
That's the best I can do to answer your question.