How transfer music from my PC to my iPhone 4?

I recently got the iPhone 4 as well as a new laptop. I do not use my old laptop at all now but I have synced all my music from my iPhone to my new itunes on my new laptop. Earlier today I downloaded more songs and I can't sync them to my new iPhone without erasing al the music and playlists already on there. I really need a solution to sync my new songs onto my iPhone because when I check the box to manually sync songs the option of erasing still comes up.

ITunes

Well ofcourse you can just use itunes, but if you're device is jailbroken, and you we're so stupid to NOT install appsync, so that whenever your device is connected all your illegal apps disappear, you can use: copytrans control center, just google it, its free (for music only).