A solo exists in the interaction between you and the other musicians. It's not all about "you" and "your solo." It is the music created by this interaction as you tell your story through improvisation.
I saw a wonderful advert a few years back. It read: "Don't just talk, tell your story."(I forget what was being advertised). This, to me, sums up so much of what is involved in improvisation. You must have something to say. Regurgitating licks doesnt count
Aim to tell one story when you solo, not a whole bunch of fragments of different stories which are unconnected and don't amount to much. (See comment above about "licks"). Melodic development is important!
You are not just playing over a bunch of chords. You are playing a song. Your improvisation should reflect this.
One of the incredible things about improvisation is the element of surprise. Neither you nor the audience knows what you are going to play next. There is beauty in this.
The audience wants you to take risks. They want to see how you recover from "mistakes." Walk the tightrope, be fearless.
Some useful things:
Scott Henderson "Melodic Phrasing"
Victor Wooten "The Music Lesson"
Stephen Nachmanovitch "Free Play Improvisation in Life and Art"
About the Author:Dylan Kay is a professional guitarist and teaches guitar lessons in Auckland, New Zealand. He can be contacted through www.AucklandGuitarSchool.co.nz.