just start playing something. My best solo's come when I'm just messing around practicing. My solo's tell a story in themselves, like the song. What I mean is for example, one of my solo's for a song I wrote starts with slow vibrato bends up and moves up neck where I'm playing loosely around an A dorian Minor scale, then hammer ons to an A minor arpeggio, then another minor scale run bleeding into an onslaught around A pentatonic Minor.
When you finish hearing it, it sounds like it took you somewhere, not just pointless fast playing.