A lot of times insufficient space is caused by Facebook, SMS, and Email. You can move some of your apps to SD and see if that helps but the things that usually make the biggest impact is clearing the cache for Facebook, deleting large volumes of MMS and SMS messages, and deleting sent emails from your device storage.
To clear caches go to Settings>Apps>All scroll to FB open and manually clear the cache. You may also want to try this for any of your other apps along the way, it's just that usually the FB app manages its cache poorly.
Obviously deleting hundreds of SMS and large MMSs is a good maintenance practice. If there are things in there that you just cannot live without, check to see if you app offers backup or caching of some kind. It should be in the settings. If not there are apps specifically to back these up and other general backup apps that offer the same.
It seems, and I'm not sure why, but email apps seem to want to save your sent items locally on your device. If at all possible change this setting so that future sent emails are saved to the sever. Maildroid is one that I know for sure allows this change.
Insufficient space is usually caused by poor resource management more often than actually being low on space, and these three are the most likely suspects when it comes to that issue.