By far, the easiest way to defeat me in randori is to wear me out. I am 36, 230 pounds, and don't work out enough. Lie on me and make me work - I'll be wiped in no time.
Recently though I have been training with the kuji-in. Funny, that. It seems to work. The Bansenshukai curriculum has one of the nine ninja mudra for each of the nine kyu. The first mudra, for ninth kyu, is Rin or Strength. Every day, during my meditation, I have been focusing on those times when I have found extraordinary inner strength. Once, I stayed up for a very long time when I had a project due and Adam was sick. In a less poignant example, I recently ran a long time on the treadmill, finding some inner endurance. I lasted a long time in one particular randori, and can recall that.
All of this is done while making the Rin mudru, which is hands together, fingers intertwined. The middle finders are raised and touching. what this allows me to do is use the unusual muscle memory to recall all of that hours of meditation in a moment's notice. Say, for instance, right before a long randori match.
It might sound dorky but it works. Just like sitting in Seiza might make you think about class, the mudra recalls the topic of the meditation. I was impressed. I am totally not into mystical stuff (ask anyone about me lack of respect for religion) but when something works it works. The tie between muscle memory and long term memory is well documented.
I like it a lot. This is where the 'magic' of the ninja comes in.
More at Wikipedia.