On many occasions of running I have had the overwhelming feeling of just wanting to stop and walk. These feelings can happen at just about any time throughout a run, even within the last mile of a planned route. I have tried mantras, cursing,and many forms of self-bribery, usually with fair to poor results. One particular motivational technique I have been using lately that seems to be working well is to throw in some fartleks. That's right, when I feel myself getting bogged down and wanting to stop, I will start to run harder. It may sound strange, but it works!
My advice is to pick a point up ahead such as a sign or the end of the block and run hard to that point. Treat it almost like it is the finish line of a race, you don't have to sprint the distance all out, but give yourself a good hard push. Not only does this clear the "cobwebs" out of my legs and get the blood pumping, but it helps with the mental battle too. Remember, the faster you run, the sooner you can stop running. When you reach the point you were aiming for, back off to a nice easy recovery pace. Remember, the point is to shake things up a bit, not to cause yourself a heart attack. Use the same technique for the recovery portion of your run, pick a point ahead of you, when you reach it, dig in and run hard to the next selected spot and repeat. You can use this technique as many times as you like throughout your run.
So with this technique, not only am I making a physical change during the run, but I have mental justification to back up what I am doing, therefore "busting through" a tough spot during a run. An side benefit of doing this is that it should eventually make you a faster runner because of your increase in pace.
Good Luck and Happy Running!