Knowing your maps is a key part to this game. Learning where everything is on the map is a long process. Once you get the basic layout down start looking for vantage points and angles from all areas of the map. Look for things that provide you cover and still allow you to shoot without being seen right off the bat by the enemy. Once you learn these things your gameplay will improve. The rest is just general combat common sense. If you kill someone expect them to come back to that area to look for you, so watch your back and look the other direction in case they come another way to try and sneek up on you. Stay away from snipping zones and look for ways to see those snipping zones without being seen. When you don't move around then you will probably get a rocket or tube in your face to get you out of that spot.
Some people are good at running and gunning. I am personally not. I find it better for me to crouch and walk. Don't get me wrong there are times when I will need to run but it is something I try not to do a lot of. I don't like running cause
1 most people can hear you coming 2 I can't get a shot off fast enough if I am aiming down the sight or hip fire also3 it just seems reaction time is slower when I am confronted by an enemy which keeps me from getting a shot off fast enough.
Now I know if you use certain perks, running and gunning is a lot easier. That is probably why a lot of people use light machine guns when doing this. Cause I have noticed they are lighter than all the other guns, like assult rifles. Walking is even a little faster and so is reaction time when it comes time to shoot the enemy. But I mainly run the following perks:
blind eye/proassassin/prodead silence/prostalker/pro
In my experience using other perks just gets me more deaths than kills so those are what I usually roll with.
So happy gaming and hope this helps some of you