Ceano : Try to find a officers uniform, with a simple soldiers uniform much more ennemies will recognize you as a spy.
Even with a officers uniform some officers will detect you, in a way it makes sense, in real life those guys often know each other, thats why i like this game so much , i find a lot of stuff really good.
I know its not easy to find a officer willing to lend you his uniform
Use the ingame map a lot to detect the field of view of potential dangerous (red) ennemies.
I find this ingame map is something wich his missing in Hitman, you never know when you will be spotted or not.
I found out that a not to nearby soldier will find the body of his mate much quicker when you stay close to the body, again makes sense, when someone's looking at you, he'll look at the surrounding around you more intent then if you where not standing there, after wacking someone, move away from the body as quickly as possible.