The temptation is to let your fingers do the improvising. It becomes a mechanical rather than a musical exercise, and you end up just throwing out random bits of scales, or following standard paths. You can find interesting things this way sometimes, but really you need to be letting your mind wander musically, with the fingers following rather than leading.
Know your scales, and know them all over the fretboard. You don't need to learn them all at once - pick a key and learn that scale everywhere, and know how to move around the positions. Learn the actual names of the notes, and be aware of them as you play.
The key to improvisation is to be able to play by ear. Take a melody and play it. If you can't do that reasonably proficiently then you won't be able to do anything but random noodling when you improvise. Take melodies as complicated as you can handle - jazz tunes with strong melodic lines are great - and then start adding other notes here and there to make simple harmonies.
Once you can do this by ear, you're ready to head off on your own.
When improvising, you still need a tight structure to improvise over. Choose simple chord progressions at first - your I IIm Vs and so on. Try to emphasise the chords rather than the melody at first - otherwise you'll get lost easily.
Try not to improvise endlessly - go around your progression twice, and try to get to a satisfying finish, and then just strum your progression twice through. Repeat a few times, but again, try to reach a definite end.
Well, that's my advice.
