I would install this app:
Also make sure all your apps are moved to your SD card with this app:
https://play.google.com/store/apps/details?id=com.a0soft.gphone.app2sd&feature=search _result#?t=W251bGwsMSwxLDEsImNvbS5hMHNvZnQuZ3Bob25 lLmFwcDJzZCJd
If that still doesn't help then you will need to delete some apps. Also if you have a lot of text messages just sitting in your inbox I would try deleting those too. When I had my Nexus One this was a big issue and the only solution (besides rooting) was the above. Good luck.
Past Devices: Google Nexus One (sold), Samsung Galaxy S2 (sold), B&N Nook (returned), Samsung Galaxy Tab 7 (sold), Samsung Galaxy Tab 10.1 (On the Shelf)
Current Devices: Samsung Galaxy S3 (AOKP), Nexus 7 (Stock), Chromecast