While I don't write my own music "officially", I do find myself whistling or humming solos over songs I'm listening to. It might work for you to do the same in writing them. Listen to the song you're trying to solo over, the solo isn't a bit you just "tack on" to it - it's built from the song, it should enhance what's already there. So listen, and just try humming or whistling over the progressions and see if you can find something that you feel fits.

I'm sure other, more experienced, players and writers will have other methods. I just figured maybe this might help ;D