1. Check to make sure USB debugging is enabled on your phone.
2. Connect your phone directly to the computer; connecting the phone to an external device such as a hub or a cradle may cause a disconnect.
3. Check to make sure that the USB cable is securely connected at both ends.
4. Reboot your phone.
5. Check if USB driver is up to date.
6. Try to use a different USB cable or port.
7. Update the device software.