"Whistle Stop" said:Rick's mention of harmony singing with a partner reminded me of a tip. If you don't have a partner, but want to learn to sing harmony, a great place to start is by listening to the Everley Brothers. First listen to what they did, then try singing along with one part, then the other, and get comfortable with how they fit together. Then when you're ready for the next step, try adding a third vocal line of your own. The songs are "standard" enough that it is generally apparent what is "right," but they're inventive enough that it's not a cakewalk. Start with the easy ones -- Bye Bye Love, etc. -- then move on to Devoted to you, Crying In The Rain, etc. Guaranteed to develop your ear, and your singing prowess.
There are other people out there that may be equally valid to do this with, but it's hard to go wrong with the Everleys -- and the fact that there's only two of them means that there's always room for a third line.
Another duo that are good for this is Ian and Sylvia - the recordings from the 60's are mostly in reissue on CD. An additional advantage, that I discovered when one channel on my car stereo broke, is that those recordings were made when stereo was new, and they didn't really know what to do with it yet, so they recorded one of the vocal parts on each channel, rather than the whole business on both.
That means that you can use the balance knob on your stereo to listen to just one part, then just the other. A great study aid. (Unless they remixed for the reissues.)
Peter.