If you are talking newer songs, you should try to find the acapellas for them, if you are talking older songs, it depends on how they were recorded, alot of older songs primarily from the 60's and 70's were mastered on "3 channels", the drums usually in the left channel, the melody and bass in the right channel and the vocals in the center channel (you can tell these types of songs simply by listening to them with headphones), these types of songs are easy to sample the vocals using a program like adobe audition using an effect called "center channel extraction". Otherwise it's generally difficult to get a clear sample without it sounding all "phasy".