stirrups off, think of moving your hips back and forwards like a pendulum instead of up and down and sit up tall to get the hips to absorb the movement, not higher up your ribcage.
someone lunging your horse to start may help so you can just concentrate on your position
the easiest way i've found is to sit an extra beat every other time in rising (so: up, down, down, up, down, down, up) and then gradually introduce more sit beats and fewer rises, until it feels easy. this will keep you relaxed hopefully - if you try to 'lock' your seat down it can make you stiffen against the horse, which makes it even more difficult. relaxing into it and just letting gravity keep you on the horse, not muscle strength, is key.
Thanks, will try all of above! Its annoying that when you change diagonal and sit a beat it is very easy but when you try to do more it becomes very tricky. x
The best advice I was given was to think of doing rising trot with no stirrups as it gives you the feel through your pelvis and moving your core that you need. You exaggerate it at first so you really are rising, then gradually reduce the movement.
In walk and an enclosed area or on the lunge - shut your eyes and focus on the way your hips move with the horse, then move into a very slow trot, still with eyes shut - and keep focusing on the movement. If you lose the rhythm then go back to walk and start again.
The movement the horse makes moves each hip individually, as one diagonal lifts and goes forward the other is going back and down - someone once described the movement as similar to riding a bicycle. My sitting trot improved no end with this approach. Once you feel more relaxed then take away the stirrups.
Shutting your eyes helps you to focus on the movement without visible distractions.