i recomment opening up your finder window and going through everything that is in here: movies, pics, music, documents, downloads etc and throw away anything you no longer use or need including applications.
once you have thrown away any unwanted items be sure to restart before emptying the trash on the dock since that way it tends to empty faster. (also a "secure" empty of the trash is not needed, normal empty by clicking the button will do)
can also remove the user and mac HD caches as well
if you have a large iphoto or itunes library you may want to look at putting those on an external drive to free up more room on the internal HD. can also get more RAM if it's able to be upgraded further.