If you're saving JPEG files for the web or email to friends, choose File > Save for Web. Preview different Quality settings and select the lowest quality that still gives reasonable results. The following illustration shows artifacts when Quality is too low.
As an added plus, Save for Web also strips color space and EXIF information from the image for a smaller file size. If you are working in aRGB (Adobe RGB), then Image > Convert to Profile, and convert to sRGB before saving.