Many years ago I had an instructor who used to shout
"Hips to hands" when we were learning rising trot.
The rise needs to be an up and forward movement, rather than a straight up and down, but try to do this without tipping forward, just allow your hips to swing slightly. This will help you keep more control over the movement. I think it is more comfortable for the horse as well.
Also practicing rising without stirrups is good for you
- builds up the correct muscles and balance. Start with very short spells and build up - like whot I do - not
If it all seems hard work - remember there are hoards of english style riders who are struggling with the sitting trot.