im not too concerned about the K/d ratio. During a game I rarely get a 5 kill streak and usually get about 5 kills and over 10 deaths...... lol
I guess I want more kills and less deaths, and a few killstreaks every now and then.
Use Marathon and run around as much as possible and spray lead every chance you get. If this doesn't work, make a post on forums to complain about 'campers'.
VxF's advice is pretty spot on. Knowing the maps is key, followed by not putting yourself into bad situations (i.e. running into 3-4 opponents in a group, not reloading behind cover, etc). Listening to your surroundings and communication between teammates is important as well.
Then there's just some games where nothing works right. It's just best to cut your losses on those and jump back to the lobby or take a break.