Rising trot is done in the same rhytym in which the horse moves its legs. As one pair of the horse's legs land on the ground the rider sits, and then rises as the other pair of legs land on the ground. It is often difficult to keep up with the rythym at first but much of this is due to a tendency to try too hard and rise too high out of the saddle.
Rather than thinking of rising you should think of pushing your hips upwards and forwards in a gentle "thrusting" movement. The lower leg should remain in the same position throughout and so it is only the upper leg that moves allowing the hips to move upwards and forwards and then back down into the saddle. As soon as your seat is returned to the saddle it should be on its way up again in a continuous movement. It often helps to listen to the beat of the horse's hooves on the ground and count one-two-one-two in time to the beats to co-ordinate the corresponding sitting and rising.
Taken From Equine World