Here are two tips for Sony Ericsson P990i and M600i that can help you save battery power:

-Disable WLAN and 3G network support when not needed in the ‘Mobile Networks’ and ‘WLAN’ menu.

-Turn off continous search for a GPRS signal under ‘Internet Accounts—>More—>Options and CS Only. You will still be able to connect to the internet when an application requests.