If you feel like you're losing hamstring tightness, the usual cue is "knees out," but sometimes this doesn't cut it. A useful cue for me lately is to imagine pushing not just my knees out, but my entire femur. I think of "pushing" my femurs out of my hip socket, while simultaneously pushing my knees out too. This doesn't change the angle of my knees, all it does is make my hamstrings tight. My cue at the start of the rep is "legs out." If you do it right, you'll feel a big difference. This, plus the cue to keep the bar in a slot over the midfoot, keeps everything tight and where it belongs during the rep.
Some folks use the cue of "spreading the floor apart with your feet" to achieve similar effect. I like this more. YMMV