Improving my ukemi skills has been an obsession of mine for quite a while now, and Tissier Sensei's students are certainly up there.
I'm sure Rob's response was mostly tongue-in-cheek, but I thought I'd just offer my perspective: the "getting thrown a lot" approach has done practically nothing to improve my ukemi, especially the so-called soft high falls. I was only able to achieve noticeable progress after I got a chance to train under instructors like Frank Ostoff Sensei and Jan Nevelius Sensei who have developed, as part of their wider approach to Aikido, a clear methodology for teaching ukemi. Playing with their senior students has been a great help. Still much room for improvement
Interestingly, it sometimes happens that when I get thrown by a really good nage, it almost feels like a good ukemi happens by itself. But for me this only started to take place when I got some important issues of alignment, relaxation and uke-nage interaction worked out to some basic extent. The flip side of being thrown like that though is that a good nage can sometimes compensate for uke's deficient ukemi skills. So as the instructors I mentioned above emphasize, it's an important part of the learning process for the nage to gradually "let go" of the uke and let them take care of themselves.