I like to use a verbal cue. I will often say eeaassyy to a young horse when they feel like they might be thinking of rushing. If they continue to rush, I won't hesitate to actually stop them in the middle of the decent and just make them stand for a moment or 2. If you just steady them with a nice deep seat and some light checks with the reins, most will figure it out pretty quickly. I do the same thing going uphill only I take a forward seat and grab a handful of mane. I don't really know why, just force of habit I guess.