P.S. I used to have this aid to 'blending' which was to imagine that an uke's ki was being directed out of the arms which they were using, in the direction they were using them - all I had to do was direct my ki extension in the same direction.
However this still tends to focus you on one part of uke's body. Instead I now visualise a large spinning ball between by outstretched arms. The ball is completly incompressible, and if something strikes it, due to the spinning, the ball rolls around the outside so the energy is dissapated at a tangent to the ball.
This enables you to change easily between different techniques in response to ukes different (and combined) attacks. It also keeps you moving off centre, and ensures that your posture and extension are always correct.
An interesting note is that (in the footage of Ueshiba I have seen) Ueshiba never disturbs someones centre, he tends to spin someone around their own centre, or sometimes leads their centre around in a circle. Also, very noticeable as he got older was this concept applied to kokyu-nage where uke is not thrown back the way they came (which is often common practise), but actually caries on in the direction they were moving and falls over - thus the feeling of not being thrown.