Early training, threatening to rear, what i can do to prevent it?

im starting to work with a 4 year old for a nice lady at a local stable. ive ridden my whole life and have had some problem horses, but i had a trainer ther to help me. the mare im working with only knows how to walk and trot so far, but sometimes she likes to stop and threaten to rear. she has neve actually reared, but i dont want her to threaten to the point where she will. they dont lunge her before they ride her, so im going to start with that before i start riding her, is there any groundwork or anything i can do to help?