I assume I'm a decent improv soloist with a guitar. Problem is, when I do improvise I can only do single note stuff. I've many solos where the player has put in multiple notes to make a heavier fuller sound, like double stops, stuff like that. But when I solo I can't seem to make it sound right. (Probably because I don't now what I should be playing!)

Does anyone know what type of techniques I could use? For major, minor and blues.
Actually i was wondering about this to. All i seem to do is play notes of a pentatonic in different orders, and at the most i put two of the notes together and hope it sounds good, which it does alot of the time. I still want to know what else i can do though.
i think its all about the chord tones. take a look at what kind of progression/harmony you are following and double stop the chord tones from your backing. that's a good start.
Yeah this is one of my weakspots and your post has prompted me to make sure I spend some time on double stops during my daily improv practice.

I have a dozen or so at the moment that I know will work in certain places in certain keys and am semi comfortable with but when I am improvising I often want to throw some in and find I have to think too long and hard about what I need to do. When I don't think about it it just comes out crap.

Here are some of the licks I know work

In G major

B|-3/5--5\3 -as used in Wish You Were Here - Pink Floyd

or E minor
B|-3/5--5\3 as in Hey Joe - Jimi Hendrix intro

B|-10/12-12-12 - repeat three or four times

There's a bunch more licks I know but when I'm improvising, unless I'm in the right place at the right time, I get stuck. So I'm going to add it to my practice routine and if I discover any tricks and remember, I'll let you know.
Sorry to bump this, but im sure someone else has to have some other tips on this topic.
John Mayer / Stevie Ray Vaughan does alot of this, good to learn some of their songs and pick ups some of their ideas.

John Mayer's 'Continuum' album especially.