Learning licks and solos by other musicians can be helpful in this respect. Obviously you'll want to develop your own voice, but no musician exists in a vacuum and it's definitely helpful to learn and analyze (if even unconsciously) the kinds of things musicians you admire have played. Depending on your style and the direction you want to go, it may be helpful to focus on other instruments as well (I've transcribed solos by saxophone and trumpet players to help my guitar playing, and I've tried to learn chord voicings from piano players whenever possible).
In fact, it's probably helpful to spend some time with totally different styles of music from your own, to learn the width and breadth of techniques available. Think of the way Bach would develop a phrase through counterpoint, and the interaction of similar phrases. Listen to Beethoven's Fifth Symphony and the way he develops a simple four note motif over the course of a half-hour piece of music. Think of the multitude of ways this concept of development of phrases or motifs has been approached, by Wagner, Copland, Coltrane, Clapton, and so on. Every one of them produced a lifetime's worth of music to explore and learn from.
Do try, too, to think about it in terms of developing a phrase. Ideas don't often come fully formed to an improvising musician. Instead, we'll take a phrase, either invented or from the content of the tune as the case may be, and expound upon it, using variation and invention to bring our own take and ideas to the framework we're working within. I neglected to mention this at first because it seems obvious, but be sure you know the tune. We guitarists are particularly prone to learning the changes but not the melody of the song. If you have the melody well in hand and in mind, it will help you to keep your improvisation "in context" as it were, and it'll be a rich resource for melodic ideas.
There are, of course, lots of technical things to think about in the mix, ideas which can span the whole spectrum of music really. A brief selection of my favorites might include:
- Phrasing/Rhythm: Think about how your phrases develop. Are you playing straight quarter/eighth/sixteenth notes? Maybe you could introduce some triplets into the mix, or even some more complex polyrhythms if you're feeling ambitious. Spend some time on the evolution of these patterns through your phrases. Building from slower or simpler melodic statements into more rapid or complex ones is a common technique. Try that, or try subverting it. Think also about the length of your phrases, about syncopation, about using rests or sustained notes to break up a phrase. Another useful thing, particularly for guitarists (and indeed players of any instrument that isn't blown), is to try to be more conscious of your breathing when planning the length and rhythm of your phrases. This can lend a more "natural" feel to your playing.
- Melody: What shapes do you use for your melodies? If you feel like you're running scales, try practicing arpeggios, and try breaking them up, changing directions, repeating only certain notes. Imagine a contour (not necessarily a precise one) of your melody. Is it repetitive, is it ascending, is it descending, is it all over the place? Try sketching a squiggly line on a piece of paper, then improvising a melody following the contour of that line. Do it again with a different shape. Learn how those shapes can affect the feelings expressed in your phrase.
- Harmony: How does your melody relate to the harmony of the piece? Could you make a different choice that would make it more interesting? As a jazz player, one of my favorite tricks for getting "outside" is to take a short motif and repeat it some arbitrary interval away. Maybe you could choose notes from the Dorian mode rather than the natural minor over this chord. Obviously, this'll depend a bit on your music theoretical knowledge and the style you're playing in.
- Articulation: How are you hitting the strings? Are you playing legato or staccato? Are you bending notes a little, or a lot, or not at all? Obviously experiment with this on your own to find your preferences. This is an area in particular where listening to and transcribing other musicians will be immensely helpful. Why do Chet Atkins and Leo Kottke and Yngwie Malmsteen and Stevie Ray Vaughan each sound so distinctly different from each other? Don't cop out and say "equipment", either—try playing a phrase the way you think one player might, then another player.
Definitely do spend some time writing phrases out on paper, too. Improvisation is still composition, after all, and the same techniques apply. Approaching the process more deliberately will definitely help to broaden the range of intellectual tools you have to fall back on when you find your "natural" intuition for improvisation is stretched to its limits, and over time, you'll find those tools being integrated into that natural intuition.