For spinning a singles yarn (sometimes called "one ply", but with only one, it isn't, technically, plied at all!), make sure to put less twist in than you would for a plied yarn, on the order of it only plying back on itself 'gently', more loosely than you would want if it were to be 2-plied.
See recent posts on this blog for information on how to perform the "ply back" test.
Then it should knit fairly straight. With loosely spun singles, each individual stitch may have one half straight up and down and the other more bent than normal, but the knitted piece should be straight.
(posted by me on spindlers, 6/14/2001)