You need to get permission from the copyright holder, which has gotten substantially more expensive over the years. Hip hop tracks back in the 80s and early 90s used to contain loads of different samples all mashed together, but it slowed down a bit after people started suing. Nowadays sampling isn't quite as popular, and sampled tracks don't usually use more than one or two samples.

The rapper Fabolous had a hit song "Breathe" several years ago that contained a sample from a Supertramp song. The track leaked and became HUGE before they cleared the sample, so when it came time to negotiate Supertramp had all the leverage. Supposedly they got $100,000 plus almost all royalties, so Fabolous didn't make any money off the single. (Though of course he made tons on album sales).

EDIT: Just because you're supposed to clear samples doesn't mean people always do. If a sample is obscure enough or sufficiently altered, artists may try to get away with it hoping that no one will notice -- apparently that's the case with Daft Punk. I think Timbaland has gotten into trouble sampling foreign artists without permission ("Big Pimpin'", maybe), and the source of the sample in Nas's "Nas is Like" went unknown for around a decade. See http://www.whosampled.com/sample/4061/Nas-Nas-Is-Like-John-V...