I was never trained to be a salesman.
I never even imagined myself doing it until I was about twenty... then I discovered that a LOT of successful people started in sales. Those self development books you might have heard of or read? W Clement Stone, author of The Success System That Never Fails got started at around four. Dale Carneige sold bacon, soap and lard.
Chess and fiction writing are... pretty different from sales. Most people who do these kinds of things are not too hot at selling themselves or products. But the character traits you develop in these kinds of areas can actually help you to be excellent in sales. You see, and this is the kind of thing which surprised me when I started out, most salespeople don't study sales at all. They just get by on being naturally sociable people. This comes as a shock to a chessplayer. Quite comical looking back at it, in my first proper sales job asking bewildered colleagues if they had "heard of NLP?"
So... you're making a decent living (and this has been your profession for years) but you haven't heard of NLP? Ironically, some of them were using it in a way without knowing.
Anyway, forget money for a second, sales skills you develop can help you in all areas of life. Sales is a real life test of your body language reading skills, memory, vocabulary and often physical endurance. There is no hiding place. You are responsible for your results, and there is no rule that says just because you "turn up for work" you will get anything.
So anyway, as with a lot of what I do, I read a ton of theory before I even started DOING the subject. Probably too much, but it all helps in a way. Ironic thing is, if you're doing a sales job for fourteen days in a row, you don't have much time to read any theory then. Which is good in a way, because you only "get good" at sales by doing it a lot. Rinse, repeat. Do it over and over and over again.
Doing this, over time you become really sharp at reading body language. Within a fraction of a second of somebody opening their door to you, you just know which tone of voice to use, which words to say. Key point here, and those of you who've read me writing about this before may have guessed this already, a lot of it comes down to being none needy.
Even if you are saying all your nice sales lines you have learnt in a book, or even been taught in the field, many people can reject you just because your body language or tone of voice is slightly incongruent. It's happened a lot to me. But now when I get anywhere near the peak of my performance, doesn't happen too much, at least compared to other salesmen.
You should set yourself goals, your own goals like in Napoleon Hill's Think and Grow Rich, not just "management targets". Short term goals work for me... I usually set myself ones that are a bit 'unrealistic' so even if I get close, that's pretty good. This allows you to keep your frame, even after constant rejection. If your subconscious mind really wants it, everything you do will bring you closer, even when dog tired. That's how it's been for me this year.