How to cut down your storage on an iPhone?

It won't let me download any more music or take pictures. I only have 600 pictures and 300 songs.

Uninstall apps you don't need/use
Get rid of media (pics/vids/music) you don't use

Also try a desktop app called PhoneClean to help clear caches.

Perhaps you could use drop box to back up your pictures to a desktop and get rid of them on your phone.clear cache/delete or uninstall some unused apps. Try using a music streaming app instead of storing all the music on your phone.