I use Apple's wireless Mighty Mouse, but it often loses the connection with my Mac. Is there anything I can do to improve its reliability?

First, check your mouse’s battery level. In your System Preferences, choose Keyboard & Mouse (in Leopard) or Mouse (in Snow Leopard), and click Bluetooth. If your battery level is low, pop in some fresh batteries.

Make sure that you’ve installed the Bluetooth Firmware Update 2.0.1, which increases compatibility with Apple’s wireless mice and wireless keyboard. You can download it from Apple’s site at support.apple.com/kb/DL889.

You can safely delete the Bluetooth network service if you're not using a Bluetooth modem to connect to the Internet.
 
If you’re not using a Bluetooth cell phone as a tethering device for your Internet connection, remove Bluetooth from your list of network services, because it may be interfering with your mouse. Go to System Preferences > Network, and if it says Bluetooth in the left-hand list, select that and click the minus sign below to delete it. Then, click Apply.

Now go to System Preferences > Sharing to see if Bluetooth Sharing is turned on. If you’re not sharing your computer with any Bluetooth devices, uncheck it to turn it off.

If none of that fixes your problem, use the Finder to go to both /Library/Preferences (in your top-level Macintosh HD) and ~/Library/Preferences (in your user folder), throw away the com.apple.bluetooth.plist files there, and restart your Mac.

If all else fails, you might have a hardware problem with either your Mac or your Mighty Mouse.