If there is one principle that is discussed and trained extensively, perhaps even obsessively, in aikido classes and some judo classes, it is the idea of ma-ai.
Ma-ai is usually thought of as a boundary around your body, the inside of which is defined as your personal space. So long as uke is outside of ma-ai, he cannot touch you without first moving toward you. If you allow uke inside your ma-ai then you cna potentially be attacked without having time to respond. Typically, ma-ai is thought to be the length of your arm plus the length of the attacker's arm, but this distance can flex a bit under various circumstances.
Despite our tendency to obsess about ma-ai, it is easy to get lax in your thinking and practice. This is something that you have to watch out for in your practice. A good way to instantly remind everyone about ma-ai is to put a rubber training knife in uke's hand (or in tori's). You dont have to change anytihng else about the practice, but as soon as someone is holding a knife, ma-ai becomes more obviously important.
A very good expression of ma-ai that is used by the Boy Scouts to teach knife safety is the idea of the Blood Circle. A blood circle is a radius around the knife-user the length of his arm plus the length of the blade. Obviously, anything or anyone within this radius is at risk of being cut if the knife-user slips.
At our house, my oldest son has reached pocketknife age, so we've been talking in terms of Blood Circles a good bit lately. Today, he picked up a bokken to do some jodo, held it at arm's length and turned in a circle and said, "Look, Dad, A really big blood circle!"
I smiled, and promptly the jodo lesson of the day became an emphasis on the interplay between uke's and tori's blood circles.
[photo courtesy of Hani Amir]