What has worked for me is enabling SMART REFLEX via the terminal commands.
I used to get 4-5hours with moderate to low use. Now i get 14+ hours with heavy to moderate use. Maybe an hours worth of phone calls and TONS of texts, Hundreds of emails(just enabled nokia messaging to see if that helps), about 30minuts of games, at least an hour of music.
Enabling smart reflex really helps with the idle time.