Gosh, this is a sensitive topic right now.
On Tuesday the first flautist (who is one of the best in our little orchestra) and I came in for a particularly hard tongue-lashing as we were sight-reading Dvorak's Slavonic Dance No.1. Our timing and pulse seemed to be a it off and we made each other worse, deteriorating as we got more and more anxious.
I could do with flobiano's sightreading skills!
I do some variations on the above. I have no idea whether it's a good thing to do or not but last year I learned my Fireworks part (a humble 3rd oboe) by playing along with a CD - without headphones, much to my mother's utter dismay (she doesn't like Handel). I also like to get a look at the full score if I can.
Our conductor (also my oboe teacher) is now insisting that we attempt to read at speed and not slow down, even for the difficult bits. He recommends taking one note at a time if necessary. Another variation on that is to play, for example, just the first couple of notes of each bar and gradually add more. It does seem to work but I must confess that at home I do sometimes slow down so I can work out how the notes and rhythms fit together. I mostly do as flobiano does, just concentrate on the difficult parts.
Another tactic for, say, a rather fast difficult phrase is to try playing the notes in a more complicated rhythm so that when you go back to the original it seems easy (

).
None of this helps the viola player's conundrum, of how to practice dull accompaniment parts , I'm afraid (except maybe the CD accompaniment idea)*. A useful approach some of us in the orchestra have is to try to meet by section or in small groups in between rehearsals to practice. That also helps with finding out how things fit together. Sometimes you think you've worked it out on your own but when you play together at rehearsal it all falls apart because somebody else's syncopation or whatever put you off the beat.
Where "something important" happens, as gedall40 says, I find I have to pencil something in on my part, at least until I know the piece well enough eg. "flutes arpeggios..." or whatever. I am told that actually one doesn't really have time to read and absorb this while playing but I find it helps me, even if it's a sort of placebo effect for my over-anxious sub-conscious.
* I discovered that Windows Media Player has an option to slow down or speed up a track and it doesn't appear to affect pitch. I haven't often used it but it could be pretty useful for practicing. I don't believe iTunes has a function like that unfortunately.