What most people are suggesting (charging the enemy) is what I have always heard referred to as a "turn and burn." This only works when you have other people with you, enough that you can make the enemy duck back behind cover with your volume of fire and intensity thereof. If you are severely outnumbered, hit the deck and take the best cover you can find and take well aimed shots until the number are less unbalanced or you are hit. As far as cover goes, anyone who read BHD might remember that a small bump in a dirt road can provide enough cover to keep you alive. Even a curb can provide a lot of cover. The best way to train for this is to do what I like to call a freeze-frame scenario. No ammo. amongst those participating designate where you would be and where the enemy is, and look for any possible cover. practice getting behind it so that you use it effectively and can be in a position to return fire. As you do this more and more, start timing yourself, or better yet close your eyes and have someone guide you to a new place and stand where the enemy is and when they tell say "go" take off the blindfold and find cover and get behind it ASAP. After each time you run the drill, discuss it: what kinds of cover did you find? how well did it work? would it be possible to find it in an op? how quickly can you get behind it? etc. When you can do this at full speed, you will be able to think tactically on your feet, and it will be second nature to find cover that most people would overlook.
yodas right and if you are ambushed and you are in the open hitting the dirt should be the first thing you do if you get behide a small patch of grass it can keep you alive till you can get out of there or till you can find the enemy and fire back or CALL AN AIRSTRIKE