This is a sneaky five letter word usually found in the dictionary between "recumbent" and "rigor mortis".
In training, if you are not certain how "stiffly" or "tightly" you are grabbing or executing a technique, then ask your partner to tell you. For example, if you are uke and are grabbing with a great amount of force, ask your partner to tell you as you ease back what they feel. The same applies when you are executing a technique. Instead of going directly into the technique, ask your partner for feed-back on how it feels to them and back off until they tell you that they feel off-balance and that you feel "relaxed" to them.
All of the other methods mentioned are excellent for at home, however, in the middle of Shomenuchi Kokyunage or Munetsuki Kotegaeshi, quite possibly a little difficult to do.
Aikido training can help you learn to relax even when you are not sure you are.