The best way to learn tactical movement is to watch footage of combat troops, whether it is old Vietnam footage, or current footage from Iraq. You will see the same thing, the troops always get up against a wall or object, scan the area, then start to move. They move from one covered spot to the next. Here is a big difference between gaming and military tactics. In combat, the first reaction to incoming fire is to drop to the ground and take cover. In gaming, dropping presents a stationary target. If you can drop and roll, or move, it helps, but the more you move, the longer you live is generally true in gaming. Never send in Marines to do the job of a 16 year old is the case here.
Always be cognizant of what the background is around you. Even in a game, the uniforms will blend in better in some areas than others. The way to do this is to look at the opponents, and see where they are harder to spot, and use those same routes.
Movement varies if you are moving alone, compared to moving with a group. Alone you either need to check