Close applications in the background you don't need. On the home screen, press the Menu key about two seconds and any applications there, click on them and then choose Menu > Close or Exit. Make sure the Browser is not left open on an active website.
Do a simple reboot on the BlackBerry in this manner: With the BlackBerry device POWERED ON, remove the battery for a minute, and then reinsert the battery to reboot.
Make sure Memory Cleaner is DISabled at Options > Security > Memory Cleaning.