There are several things that can help you free your memory. It's advised to transfer some large application that you have installed on your internal memory to an SD card. This will free a lot of memory and prevent the error from appearing. To do this, go to Settings > Applications > Manage Applications > All, tap on any application and select Move to SD card. You could also use a third party application to help you clean junk files from your phone. Use Android Informer's database to find such an application.