How do I prepare best training material for sales training and other sales related soft skills?

You have to identify the training needs among the candidates. You should know their qualities, capabilities, strengths, weaknesses,...

Of course you need also to know the business and the market

FOR being sales representatives or in sales marketing staff..the best traing that most required is to to know very well what is the product that u handle,what is benefits that may contribute for every customer who will buy..