Mental discipline, will power, whatever you want to call it comes from a strong enough desire to do or not do something to better yourself.
When i'm training hard i get real focused on my diet. I went almost 2 years once with out slipping up. When I'm off or life gets to busy to be as focused on training as i'd like i tend to mess up on my diet a lot more.
All American EFX Rep
Natural Freak for Life