Staying undetected is always a good idea. In speedball, a lot of the time, anything sticking out is likely to get hit. In woodsball, however, I usually find it better to just stay absolutely motionless, even if I'm exposed, then make a sudden movement to pop under cover.
Warning: boring story
One certain time comes to mind- One man came to a stop at the top of a hill, searching for me. I was hidden behind a rock, with my head slightly poking out. He stared what seemed like right at me for a good minute. I was scared he saw me, but I didn't want to risk the 150 + foot shot.
He came even closer, so I tucked in a bit when I got the chance. When he stopped looking at me, and moved a bit, I got a clear shot and hit him right away.