What motivates me the most is looking at other people who are more physically developed, fit, muscled and chiseled. I then use this drive, determination, and want as my motivation.
I set my self certain goals to achieve which never ends, this is how i get by my gym routine for the past 7 years i've been working out.
Say i achieve my goal, i set my self another one to next level up. I work hard to get to that goal and once i think i've reach that goal, i look at my body in place which would need improving.. it's a never ending pattern.
Another great motivational tool is listening to others around you envy your fit athletic body, getting endless compliments.. to be honest, it feeds my ego hahahaha. It makes me feel superior and the girls love a rock hard athletic body wanting a bit of touch and feel of you.