A few suggestions:
Try clearing app caches (you can do this app by app via the settings, or there are apps in the Play Store which let to do this for all apps).
If your phone allows you to, move some apps to the sd card.
Check whether any apps are using excessive amounts of data and whether you can clean this up (you can see the data used in the applications menu).
Uninstall apps which you are not using.
If none of this is enough, with some phones there are other tricks you can do if you are rooted. But that type of thing is device-specific, so we'd need to know what phone you have to advise on that.