Watch the ZZ Shuffle - Solo online guitar lesson by Jeff Scheetz from Rock Solid
One thing you see in lots of players bags when playing over bluesy rock stuff is the b5 note. The Blues scale. This is basically our minor pentatonic scale with a b5 added. This will give you an "instant" bluesy sound when used in a tasteful way. I also do some things with the lower strings on these variations.
Billy Gibbons from ZZ Top was really good at playing in the lower register. Many times when we think solo we immediately go for high wailing notes! But sometimes the best thing to play is a lower growling riff - really feels like you are in the groove instead of just playing over the top of it.