Copyright - sampling and clearing samples

The use of samples goes beyond simply needing permission to use the song. You are *also* using someone else's performance from a recording that is probably owned by yet another person.

This is something that can get extraordinarily complex. You have a number of stakeholders, and therefore, may need to get permission from a variety of sources:
-a mechanical license - enables you to use the composition that the sample is from
-permission from the person who owns the copyright to the recording itself - The master recording belongs to the producer until such time as the recording is paid for. Once it is paid for, then the purchaser of the master recording owns it. (similar to the idea that if a photographer takes your picture, even though it is a picture of you, the photographer owns that image until you buy it from them.)
-you *might* even need to get the permission of the musician whose performance you are sampling. (especially if there are moral rights that need to be considered - ex. using a sample from a RATM song in your song that is in support of oppression would clearly be problematic)
-and if the song gets played on the radio, for instance, you need to have an agreement in place where the creator of the work you sampled from gets a portion of the performance royalty. (though this might be part of the fee agreement)

Here is an excellent article from a Canadian entertainment lawyer, who explains it in language that most of us musicians can understand:
http://www.zvulony.com/music_sampling.html

Another UK-focused article that is very good and makes reference to the famous Under Pressure/Ice Ice Baby case:
http://www.musiclawupdates.com/articles/ARTICLE%2003thesongremainsthesame.htm

Between the two of those, I really don't have anything to add, and these links explain it as well as or better than I could.

A couple of short paragraphs from another Canadian entertainment lawyer, written in very simple English.
http://www.sandersonlaw.ca/FAQ.html#A20

A move that many artists have started to choose is, rather than sample, to re-create their own version of the sound. In other words, don't use the original recording of Joe Perry playing the riff for Walk This Way... hire a guitarist to come in and duplicate the tone and performance as close as possible and use that. The word they use is to 'interpolate' the performance.

Other solutions:

1. Check sites like royaltyfreemusic.com and other sites that offer music that may be used freely (ie. without requiring the payment of royalties).
2. Check the Creative Commons database for similar.

In either case, be sure to read the terms of service for both the site and even the specific track, if applicable. You'll find that some sites and some tracks (many, in fact) will have limitations around how the work is to be used. Many works are made free for personal use, but do not allow commercial redistribution (which would, of course, be a barrier to the use of samples).

The good news is that the artist/publisher will generally be easy to contact, and may well be receptive to negotiating a deal that you can both happily live with and will not be financially prohibitive.