Have you tried turning down the tx power on your APs? It will help your devices decide to roam, and it may not actually reduce your effective range, because often times effective range is limited by tx power on the client more than the AP.
I have, doesn't really help very much unfortunately. Setting an RSSI threshold on the AP can also help devices roam, but its hard to set it at a level that works for all devices (since different devices have different sensitivities so an RSSI threshold that works well for my phone might cause some other device to constantly get dropped).
