Keeping Verticals Vertical?

Hi All,

I,'ve been working on the Trumpeter HMS Dreadnought. Although I'm almost finished with it I need to say that I had a lot of difficulty keeping the supports for the various deck levels at a 90 degree angle. My question,for my own future reference is, how do you make sure vertical supports stay straight at a 90 degree angle? Or do you just (like I did) get them "close enough" and then move them with pins, toothpicks or whatever into their locator holes.