use shift a lot, its the default button for walking rather than running, it improves your accuracy in the same way crouching would, but using crouch all the time isn't reccomended, using either the cv-47 or the m4 rifles also make sure you tap-tap, so dont hold down the button because this is definately not the best way to go about it and you'll probably be do more damage to the coffee cup at the other end of the room rather than the guy about a foot in front of you ^^, and for the most part someone already pointed out teamwork, that is a must !, being lone wolf is pointless and just ends your fun quicker, oh. and make sure you use your sound !!!!!! a pair of headphones and listening out for footsteps is the most effective way of out thinking and out playing the other team than anything else.
hope this helps buddy, css is a game you have to play to learn, these are really just a few basics, and ive played for a good few years and played at the I-series lan events and in a few good ED and ESL teams. all the best mate
sharpy.