A good exercise to begin cementing the concept of ma-ai, as well as movement from the center, is what I call walking the stick. Take a jo stave and notice that it is probably fairly close to 2X your arm length. That is, a jo is about ma-ai length. Stand in front of your partner with the end of the jo on your hara - the pit of your stomach just below the knot of your belt. Your partner sticks the other end of the jo in his hara and you both take your hands off the jo.
Now you have an absolute, explicit connection between your centers. Begin moving forward and your partner has got to retreat to keep from being poked. Start with slow, rhythmic tsugiashi and progress toward free-flowing taisabaki. Move forward, backward, left, right, turning steps, circles, etc... matching each others' motion in order to keep from dropping the jo or poking each other. Sometimes you lead and sometimes they lead. Sometimes we will do this exercise with a stick between centers and measuring ma-ai with our hands. This tends to make an even more rigid connection between centers and make it easier. For more of an interesting challenge, give each partner a full cup of water to carry (without spilling) as they do the exercise.
In this exercise it is both partners' job to not let the jo drop and to not get poked. Both partners are actively trying to synch to the other.