My horse gets MASSIVELY excited and rushy about jumping! I set up a one-stride double (don't really have enough poles to make a proper grid) and just circled before until he was calm, circled after until he was calm, repeat. It worked pretty well really, although he was still pretty quick heading back towards the baby (he is very possessive of her & to add jumping to a strong desire to be OVER THERE NOW PLZKTHX is ermmm, not the greatest idea)
Also, his problem is NOT lack of dressage training. He is very experienced and educated, good laterals, good transitions, and beautiful lengthening. He shortens his stride off seat alone and we always transition downwards off seat and then, if he isn't listening, a light rein aid.
I actually set up the one-stride double to work on ME and improve MY position, it was just there and I remembered what I'd read about grids helping with getting strong. It's only 2' or so at the moment, I'll probably put it up a bit.
Just confirming here, getting strong is VERY different to rushing! Rushing is a lack of confidence and signified by a very uptight horse that is likely to over-jump and possibly stop or run out. Getting strong is a very excited horse (also uptight but in a different manner) that will not stop or run out and is less likely to over-jump.