The top three sales management mistakes are:
Confuse the salespeople about their territory or areas of responsibility
Frustrate them by confusing their customers
Change their compensation in any way that is random or short sighted
If sales managers would focus on these three things and get them right - I mean really right, they would make their jobs easier, their salespeople happier, and generate more sales, so no more....
hedging on commissions
changing sales team structures at the least blip in sales
making good potential clients house accounts
sending other sales people or managers to customers without coordinating with the sales guys
telling them to sell or focus on one thing and then changing direction arbitrarily
hiring and firing with no real thought - salespeople as commodities
sending mixed marketing messages that force the salespeople to explain what the heck is going on
half way starting marketing 'campaigns' and expecting the salespeople to pick up the slack
After all the only thing standing between you and your customers is your salespeople.