Before you begin work on the solo, you have to know the progression solid. When you create the solo, pay attention to the chord changes in the background and try to target those chord tones in the solo when that particular chord is being played. Done correctly, the solo will 'fit' much better. Also, don't just knock out notes at a constant rate. Think of things you normally wouldn't think of - rhythmic patterns wide leaps and intervals, harmonic sequences, etc. A good solo is usually very melodic (especially important for ballads), not necessarily blazing fast (although speed really fits sometimes), and makes sense rhythmically. It takes a long to time to perfect the craft of soloing.