I believe you can encourage roaming by disabling the lower data rates on your APs. The idea being that as the device gets further away from the original AP and the signal drops away, it will be unable to drop to the lower data rates on the AP (because you've disabled them) and roam to the a closer AP which can continue to support it on the higher data rates.
At least thats what I was told on my CWNA course last week, I've never personally used the technique myself.
