The best tip I've picked up was from a Bas Rutten DVD actually. I was having what seemed to be a similar issue -either letting my opponents escape during the transition, or flat moving myself into the wrong spot. The solution for me was to trap their arm first in an almost Kimura like lock. There's absolutely NO pressure or threat of a Kimura - but it locks up their arm and gets their bicep and even arm pit sometimes pinned against my body so deeply, that I can take my time going through the transition and still hit it. I'm noticing that if I use my legs right and squeeze everything together during the transition, they can't really even hold onto their own hand for defense.
From mount, I attack their right arm by grabbing their right wrist with my left hand from above. With their elbows in and their hands up, I come down from above their shoulder with my left, my left elbow still up by their ear -almost like I'm trying to use their own arm for leverage in a "forearm in their throat" kind of thing. Then I slide my right hand like a knife under their elbow and grab my own left wrist. I stay leaning into the whole thing, and pull them up and in as tightly as possible. As you transition into the arm bar at this point, you may feel a little akward because your left hand is on their wrist upside down from the standard arm bar leverage grab, but that's part of what helps set the whole thing up from the beginning.
Of course, for full disclosure: I wear a Gi - but I think it only becomes an advantage to my opponent in their defensive options. I ignore the Gi's when I try this technique. I am also a medium sized fish in a very small pond; perhaps others with more experience will steer you in better directions!