Firstly master the sitting trot and learn to move WITH the horse and recognise its rhythm. Try counting 1, 2, 1, 2, etc.
Don't focus on sitting down and raising up, the harder you try the more your likely to lose rhythm and balance. Let it come naturally, be patient and most of all RELAXED.
Practise makes perfect!
Lastly, I was told rising trot was invented for carriage horses when a rider had to sit on their back as well as them pulling the carriage. The whole point of the exercise is to relieve a bit of your weight from the horses back to make it easier for them - not to make us riders look pretty!
An amazing rider once told me that the best rising trot is when the person finds a medium place (neither standing fully in the stirrups or sitting deeply). Let the horse's momentum throw your bum out of the saddle (which will happen) and then simply let yourself return to that position only to be thrown out again.
Eventually it will work on instinct and your muscles will know what to do without your mind even having to think it through! If you are having trouble with balance then try staying in a standing position. Keep working at it until you've learnt to hold yourself. If your having trouble with maintaining the trot because your getting sore/tired then don't stress yourself or your muscles. Do little at a time until you've built up your strength.
Hope this helps at all,
Lots of luck
19 January 2012 22:45