The first, simplest and safest step is to delete dumpstate/logcat files.
Dumpstate, logcat files are stored on the /data/log partition that, unless you root the phone, you and none of the apps you install will have access to.
The following method does not require you to root the phone or install any apps and will probably free up more memory that any other method.
1. Dial *#9900# - This will bring up the following screen - Sysdump
2. Hit the 'Delete dumpstate/logcat' button. Hit OK on the confirmation dialog.
3. Scroll to the end of the screen and hit 'Exit'
I've seen about 800MB freed the first time I used this.
As a next step I'd recommend the 'Clean Master' free app to manage your cache, data, moving files to SD card etc