Re: Taking the high road
The first line of defense is always trying to remain upright, you're exactly right about that. Pretty much any technique can be used, whichever best counters your opponent's move. The simplest thing is just moving out of the way before he can grab you -- say, tsugiashi and irimi or irimi tenkan to keep you out of the way and put you in a position to do irimi nage or whatever other technique you want. You need to practice counters so you can flow into a new technique when he blocks out the ones you're trying; and you need to practice counters to grabs so you'll be able to turn his moves against him. You have to watch out for single- and double-leg takedown attempts, too; and there are surely some judoka out there who'll try to grab you around the waist and suplex you, so maintaining ma ai to keep from being grabbed securely is a great idea if you can manage it. And you need to practice getting loose and back up on your feet before he can mount you or pull you into his guard or otherwise maintain an advantage when he manages to take you down. One on one randori attacking with a wide variety of techniques is a good way to practice this sort of thing.
But if you're trying to end a real fight, at some point you're going to have to secure your opponent since otherwise he'll keep fighting unless he's unconscious -- so to do that, you're going to have to take HIM to the ground (with koshinage or shihonage or kote gaeshi, e.g.) and will probably hold him in a joint lock while you're in suwari.
You asked a great question, though: what techniques do you aikidoka use against someone trying to take you down? I'm looking forward to hearing from those of you who train that way.