Since I didn't have a big budget for tapes and since I did this in the pre-iPod days, I listened to audio cassette programs. The 3 main ones I listened to (dozens of times each) were:
Lead the Field by Earl Nightingale
The New Dynamics of Winning by Denis Waitley
How to Master Your Time by Brian Tracy
Each was about 6 hours long, so I could complete one program roughly every 3 days. Keeping my mindset in the right place was crucial. I didn't listen to these programs for the ideas -- I listened to them to maintain the right attitude.
These days I get audiobooks from Audible.com and typically go through about 2-3 per week, depending on the length. I just finished a 20+ hour one called Spycraft, and today I started a 7-hour one called Smart Pricing. I listen to them on my iPod at the gym and while driving, and I listen on my iPad at home (while cooking, doing physical tasks, even while shaving). I always listen at 2x speed, and I really wish the iPad/iPod had an even faster playback setting.
Nowadays I tend to skip the motivational stuff because it's like listening to repeats, and I go for biographies, true stories, business, or science books. I like listening to stories of people who've done interesting things, like starting cool businesses, inventing things, making new discoveries, etc.
This daily mental programming helps to counteract the forces of negativity, mediocrity, and sheer stupidity that we get bombarded with as a matter of course.