my ipod doesn't match itunes songs

Anyone have this problem? I synced my Ipod a few months back, and after doing so it showed I had NO music on my Ipod. They finally reappeared. However, now the music in my Ipod doesn't match my 'Library' in ITunes. I purchased more music today, and 'synched' my Ipod again. However the new songs I purchased, and the music in my 'Library' in ITunes once again do not match.

Any suggestions without losing all the music either places?