I find that running with Sam and Hill and Jeff helps even if I'm not trying to keep up with them. Just having people watch/judge my performance helps me try harder and certainly makes me want to do the whole planned set.
I also try to visualize the finish chute as I'm starting to feel tired and then pretend that someone else is halfway between me and the line and that I'm catching them.
It's sort of amazing how much I lie to myself when I'm running. I think I get away with it too. I say things like " just one more, then you can stop" and " we can take a longer recovery between sets this time" and never follow through. Ha, I'm such a sucker.
I've noticed I can get away with that too. "Okay fine, this is the last one..." and then after 2 minutes of recover I've finally recovered enough to convince myself to do one more :)