Since Aikido is about avoiding struggle and performing a technique with the least amount of effort, we must consider our roles as both Uke and Nage.
As Nage, first and foremost, it is our responsibility to protect Uke. If there is resistance, if one is off balance or if the technique just is not going as it should, just relax, slow down and find your "way." Aikido is about power - an inner power - rather than force. Try not to force a technique. As Nage it is our responsibility to smoothly establish and maintain a connection with Uke throughout each technique.
As Uke it is our responsibility to provide a realistic attack (but not overly aggressive), and then move our bodies in relationship to Nage's movement so as to be in the safest position possible, while maintaining a sense of connection.
Grabs by Uke should be firm, but not rigid. The hand and wrist will be used to control whatever is being grabbed. The rest of the body should remain loose and able to adapt to the situation.
Strikes by Uke should have good extension, without locking out your own joints. Locking out joint can lead to injuries. Always try to keep some bend in the elbows, and give about 95% of your full energy. You do not want to over commit and not be able to react.
The loss of connection is an opportunity for either Uke or Nage to commence a new attack. Therefore, maintaining the connection is of the utmost importance.
Remember, you take ukemi to avoid injury only. Uke must be able to respond appropriately to the situation. This requires much training and introspection as how to do this best.
"Free of weakness ignore the sharp attacks of your enemies: Step in and act!"
– Morihei Ueshiba