It's worth deconstructing a few thrillers off the bookshelves. Map out the plot threads and see how everything's interconnected so there are no wasted characters or other plot devices (although, saying that, one reader asked Raymond Chandler about a driver he'd left waiting in a car to pick someone up, and he said 'oh, I'd forgotten about him'
. Avoid making it episodic, where each action scene is tidied up at the end of the chapter. Make it successively worse for main character as you head for the big denoument. Give everyone hidden adgends and secret motives, so no-one can be trusted.
They're strongly plot-driven, so it's usually best to plan it out, rather than just plunging into writing the chapters. It's man-fic, so go heavy on technical detail.
<Added>Worth checking out the Crime Writer's Association site for tips:
http://www.thecwa.co.uk/index.php<Added>They are also largely Show, so avoid explanatory Tell unless it's in retrospect, just to bring the reader up to speed. Usually the role of the side-kick.
<Added>The other thing is, now the Cold War is over, US-USSR spy thrillers are out of fashion. These days it's Corporate spying, or anything featuring Iraq or the Middle East.