If you have a lot of pictures and songs, they consume much rel estate. Delete the ones you do not need. Or get an external drive to store all the old stuff you rarely need but want to keep. You could also get a new internal drive for your data and keep the original for the OS and often used software.
I found a free program called "treesizefree". It will analyze your hard drive and tell you how much space is being used by every folder. You can use a tool like this to pinpoint places where space is being used that you didn't know about. It is oftentimes surprising to me where a lot of the space goes.