Watch the Improvisation online guitar lesson by Robben Ford from Blues Revolution
For Robben the essence of improvisation is "taking a leap" and feeling confident enough in your mastery of the fretboard to try something new without getting lost or hitting a wrong note. In fact, Robben suggests avoiding playing what you already know in favor of experimenting with new positions and fingerings.
Different picking styles also give your playing different textures; playing with a pick, with your fingers, or a combination of both. Bends, vibrato and other expressions are also creative factors you should employ.