If you've used a sample in your track and you've got plans to release it then getting it cleared is vital. Many, many artists have paid large amounts of money for not getting permission for their samples before they got big.
If you do it early enough getting clearance can be a lot easier than you think. And it will cost less.
This article takes you through the process including:
Misconceptions of sample clearance
Using companies to do it
Finding who owns the copyright
Approaching the copyright holder
How long it takes
If you include a sample of someone else's music in your own work you are infringing two copyrights. The first is the copyright in the actual recording, which will usually be owned by the record company which originally released the track.
(For the rest of the article, refer to http://www.bbc.co.uk/radio1/onemusic/legal/sampleprint.shtml )