Adding a bit...
As Ram says, roaming decision is always, without exceptions, made by the client. It can't be forced by the infrastructure.
A client NIC's purpose is to maintain network connectivity. It will not roam when it detects an AP with a stronger signal, it will roam when the RSSI of the first association falls below a certain threshold. This threshold is determined by the NIC manufacturer. For example Intel Wless NICs have a setting "Roaming aggressiveness" which is by default set to Medium - this setting changes the threshold. The methods Ram described are a way of making the RSSI fall below the threshold faster, thus forcing the client to search for a stronger signal.
What you are looking for is called "Fast Roaming", and there are several methods to achieve that. With MSM products we use Opportunistic Key Caching.
When the client first associates with an AP, during EAPOL exchange a Pairwise Master Key is created. The client stores the key identifier (PMKID). When the client roams to another AP, it does not have to re-authenticate, it just provides the PMKID to the next AP and the AP will allow the client through.
This feature is available for non-Access-Controlled VSCs that use WPA2. When you look at the VSC settings, you'll find "Fast Wireless Roaming". The APs also support Spectralink Voice Interoperability for Enterprise Wireless (Spectralink VIEW), you'll find this in the AP Radio settings.