If you have not logged out or restarted in a while, that could help too as the restart will delete a lot of temp stuff (swap files I think).
You need to have at _very_ least 10% of your HD free (some even say 15-20%). If you have less than that, it is likely that you already have damage on the HD and you should think about running a utility like TechTool or DiskWarrior to make sure it's OK. If you don't have them, back up your data and think about a re-install.
A bigger HD is a good suggestion too.