I used to be in a similar boat, with a bit under 300GB of music, until the great music-cull of 2009, when I deleted the whole lot and started from scratch. These days I keep it under about 6GB - if something new can't fit on my phone, I delete albums until it does. This has been pretty tough at times, but it has the advantage that I don't have anything in my library that I don't absolutely love :)