Use camoflague and concealment to your advantage.(Obvious)
Stay low and with a partner covering you, pretty much "leap frog" down the field. I did this against a fort and we bunkered 3 people and we were the only two left on our team (4 vs. 4), they had no idea where we were.
Don't make noise (I'm really good at stating the obvious)
Attack the flanks hard, this takes the opponent's attention off their middle.
Use the terrain, ridges and ditches make great trenchs, hill are great to shoot down from and really crappy to advance up. Just be sure not to expose yourself onto of the hill by standing straight up, always stay low.
Instead of going out and wearing a hot ghillie suit, a christian fish shaped branch, with twine tightly strung through to form a web and local brush and vegetation stuck in works very well in hiding you, especially in low light situations. (See www.norwich.edu/flc/photos/pages/Camoflague.htm
, its in its beggining phase)
The best defense is an offense!!!