Gen has it nailed. I have had huge (Howard Huge) feedback from a synthetic solo structure I call "mixing it up" which uses a combination or major and minor pent scale segments.

Disclaimer - I say the following only as a starting point. Every true guitar solo should be a conversation, not a teacher's homework assignment......
For basic 3 or 4 chord backs - Major runs are generally downward (notes run from the top of the scale down, but not necessarily straight down, give it some peak and valley to avoid the chromatic scale exericise sound Gen mentions), minor runs are basically upward. Alter the application of each across (a short) one or two bar sequence, while climbing up the neck on each application (the build). OR -You can combine two Majors (one run down, move up the neck, one up), then two minors (up, move up, down, whatever), etc.. Feel the breeze.
Repeat the basic structural patterns in opposite to round out the solo. Lost? Think of this in "chunks" or "inserts". Take your minor pent solo, remove a fragment (a bar for every 2 bars) of the minor set and insert a major scale fragment (without abandoning your "topic"). Keep general scale activity up then down, which will add a fluid feel without creating a noticable transition between the two (but highly "conversational"). Still lost? My fault - it's hard to explain without dumping an MP3 or lengthy tab on you. Take Gen's lesson.