Try not to allow the legs to come forward or the back to drop too far back into your hands, this posture should ideal happen with the hips and legs in line, on a plane as it were.
In Urdhwa baddha konasana, I find it useful to roll on to the outside of the feet to open the legs wider before bringing the feet back together.
There is the option of bringing the legs down towards the mat and then raising them back up into urdhawa konasana, three times on the breath, each time going deeper into the fold.
Engage bandhas, drawing the belly back and up to create mere space for the deep forward bend.
We can begin by holding the toes, then the sides of the feet and finally the heels, drawing ourselves deeper into the posture on each long exhalation.