Thinkertoys by Michael Michalko. Attend four sales seminars a year. Invest in yourself. Learn new things and meet other learners. Listen to sales tapes twice as much as listen to radio in the car.
Repetition is one of the keys to mastery.
The other is practice what you learn as soon as you learn it. Record yourself reading a book on sales. By recording a book yourself, you will master the sales skills and hear your presentation skills at the same time. It is a powerful exercise. Read and record 30 minutes a week.