you should try and see Henry Kono at some point (up in Toronto but visits the US sometimes). His training is ALL sensetivity training and no 'technique' though amazingly the techniques drop out very easily. Very hard to explain in writing but I'll try (apologies for length of post):
Main concept: when nage makes contact with uke the centre is no longer his own centre (hara) but the joint centre of the two people.
try: nage has flats hands together in front of waist (arms bent but extended in relaxed way). Uke hold nage's hands and pushes directly forwards. Nage moves gradually back and to the side ensuring consistant contact with uke. When nage turned enough they can push in the same direction as uke is pushing (like a very slow sumi-otoshi).
Also, uke can try pushing upwards, downwards and nage follows exactly. It is imperative that nage keeps hands in same position (in front of center line) and moves their feet and body only. Also, nage must stay upright (bending knees only). With can result in irimi-nage, kaiten-nage type techniques. Nage can raise and drop arms (together) but only in response to uke's motion.
Try: same thing but with nage's hands apart (can get more realistic sumi otoshi, ikkyo)
Try: ai-hanmi grab, uke pushes with one hand. Nage first moves to inside (slightly) in circular backward motion. Then they move all the way around the shared centre point (point of contact), to get to outside of arm - it is very important that uke pushes slowly and consistantly and nage neither pulls nor resists (a small consistant pressure is best). This position is then Ok for ikkyo - don't force, but continue by holding uke's elbow and wrist and walking forwards in same direction as uke.
ALL aikido techniques can drop out of this type of motion. The important thing is to keep the hands in the same position in front of your body (except can move up and down) and to move your feet rather than trying to move your hands (hands are ABSOLUTELY responsive to uke). With the ikkyo technique it is easy to see if it is done right - if uke feels a pull or push they should resist (if nage tries to force ukes arm up it is easy to resist since nage is using arm and not body). Can also try with flat of hand for contact - thus if disconnection it means nage is either pulling or pushing.
Hope this helps, hard to explain without feeling it.