Keyword Density According to Google
Google’s Matt Cutts recently addressed a question about what the ideal keyword density is. He refers to the diminishing return of repeated keyword usage and the danger of keyword stuffing which can earn you an over-optimization penalty.
Matt says it best, so take a look at the video.
Increasingly employing variations on a keyword phrase rather than strict repetitions of a verbatim phrase seem to work best.

Long tail keywords don’t refer to monkeys, but to the typically longer
One of the worst things to ever happen to web sites was the discovery of keyword density. According to copywriting expert 