Delete stuff from the system memory. No joke. My system was starting to get a little bit laggy (especially messages app), so I deleted all of my messages and a load of apps I no longer use. As well as my phone speeding up by leaps and bounds, my battery is getting about 3 hours additional life (minimum). Before, 15 minutes after taking my phone off charge it had lost a bar, but now.. I took my phone off charge at 7:15am and listened to music for 40 minutes on the way to work.. it's now 10:19am and I still haven't lost a bar!