iphone out of memory space--how do i get rid of some of my music

i have >2000 songs on my i phone, along w/about 100 apps and now am out of space.

i want to get rid of some of my songs to free up some memory, BUT i don't want to lose the songs from my itunes library the next sync.

how do i fix it so i can do the following:
1. get rid of some songs on my iphone
2. not lose any songs in i tune

if i delete them from my iphone (can i even do that?) will it then erase songs in my itunes the next time i sync?

i have iphone 3gs w/the os 4 upgrade