Forcing Carrier Switching:

The phone can be forced to search for service on any of the three by inputting the following dialer codes:

*#*#34777#*#* – Force Sprint
*#*#34866#*#* – Force T-Mobile
*#*#34326#*#* – Force US Cellular
*#*#346398#*#* – Force Next Carrier
*#*#342886#*#* – Resume Automatic Switching

No matter which carrier you force the phone to switch to, the phone will resume automatically switching after two hours.  At this time, this cannot be stopped.

Additionally, these dialer codes may come in handy:

*#*#344636#*#* – Current Network Info
*#*#34963#*#* – Repair a Bad Activation
*#*#4636#*#* – Launch Phone Test Menu

Dialer Code Apps:

FiSwitch is primarily meant to act as an easy way to input dialer codes.  If you are stock, the code will be copied to the clipboard, it will open the Dialer, and if you enable certain settings, be automatically pasted into the Dialer.  All you have to do is hit dial  If you are rooted, the code will be automatically executed upon pressing the button in the app.  The app also comes with a handy widget that presents the three carriers to quickly switch between them.

Signal Spy provides the same dialer code input into the Dialer, although having root will not assist you here; you still have to hit dial.  However, the app displays much more information about the network you are connected to, as well as a network history.

I personally have both.  I use the FiSwitch widget for quickly switching carriers from the Home Screen (while rooted), as well as determining which carrier I am on at a glance.  I use Signal Spy to determine which network and band I am on, as more detailed information is displayed within the Signal Spy app as opposed to just the carrier name on the FiSwitch widget.