For me its all to do with learning new maps (which can take a while) games like CoD rarely rely on uber reflexes these days, its about knowing where to aim and where to look when walking around for the usual camper spots and entrances. When MW2 came out, I sucked because I had no idea where I was going, once you start getting the lay of the land and get a gun you like then you start to rack up the kills, same happened when the 2nd map pack came out (couldne bt bothered with the first), I was aweful but slowly as the maps get put into your head then you will fare a lot better. Its the same with every online game ive played recently. I start off middle to bottom then end up near the top after quite a few plays on the map, still some blaco ops maps I always suck on purely because ive hardly played on them and get lost still looking at trees.
The general populace of online shooters are rubbish, they just know where they are going. You will improve, no need for external practice. Get a gun you like and try and work out where the campers lie (there are ALWAYS campers) and which corridor goes where. Works for me anyway.