My first option is down by the wrists. I place the sensor one handbreadth above the base of the palm of the hand. I use my own hand for this distance. This helps to ensure that the sensors are placed as symmetrically as possible. If I am unable to place the sensor there, I use the deltoid as my alternate location.
When placing sensors on the legs, I place them one handbreadth above the medial malleolus (medial ankle bone bump). Again, I use my own hand for the measurement of this distance for the sake of symmetry. If I am unable to place the sensors in this location, I will place them on the thighs if needed.