How exactly do you go about using several scales in a solo? I'm not quite sure how it's done. I know that Ritchie Blackmore did it by using different scales for each phrase in the solo to Smoke On The Water, and Slash did a similar thing with Sweet Child O' Mine (and I'm pretty darned sure those aren't the only cases). I can see how you can easily combine the Pentatonic and blues scale in a given key, since that just comes down to whether or not you use the "Blues Note" (I think it's a flat 6th, but I'm not 100% sure. I just know how to play the scale...), but what if I want to make a solo go from either a happy sound to a more sad one, or maybe from aggressive to more calm. I know the modes each have their own characteristics (for example, I've read that the Dorian mode supposedly sounds even sadder than the Aeolian mode), I just don't know what the secret trick to using more scales in a solo without making it sound bad is...

Any help is greatly appreciated.