A few things I do with writing solos is, plan out how you want the solo to sound, what key it will be in, what scale you will use, etc. A good way to get ideas is to record the chord progression, and then improvise to it. Try to do it slowly, sometimes I plan out 16th note pentatonic runs by playing them in 8ths, and then speeding it up later. Also, try to study alot of theory to make sure your solo is interesting, at least to you, try new things, tapping lines, sweep pick, and so forth. I'm all over the place with this post. Hopefully it helps somehow.