I had three method's for running motivation, and only one of them is slightly sexist.
The first method, as mentioned above, was getting yourself pumped up to excel a little bit further, surpassing your best effort and feeling like you've accomplished something. Problem with this method is that once you hit a performance plateau, it's really easy to get down on yourself and lose motivation, and the only answer to a plateau is to work through it.
The second method (and by far the nerdiest and most embarassing) is to think about why you're out here doing what you're doing. Visualize if you have too. There were many evenings out running where the only thing keeping me going was the image of a Hornet in my head, and understanding that every step I took dragged my lazy ass that much closer to that airplane.
The third method, and by far the most sexist, is to keep the opposite sex in mind. Think about it. You're out there getting your ass in shape. There are two direct consequences to running, the more important of which is achieving your immediate goals, the lesser of the two being to tighten up that flabby body of yours. Gross. My girlfriend loved the fact that over time my body was getting stronger, muscles more defined, yada yada yada... She also liked seeing me sweaty and hot after a workout. It should be noted that ot all women like this, and while my girlfriend liked it for the first five minutes after a workout. In the next five she would be demanding that I go take a shower. Vizualization works here too, just don't go overboard with it or everyone is going to wonder what the hell is wrong with your running shorts.