Here's my suggestions for improving your base skill - though perhaps not your score for a while.
1) Avoid hardcore - when any weapon kills in one bullet there's no motivation to improve aiming and tracking, you can simply spray in the general direction and often get the kill. The same goes for the stopping power perk. While you're going to struggle for a while because you have to hit people quite a bit to kill them, you will improve your ability to maintain aim, as well as avoid damage.
2) Train your movement - it's important to be able to move unpredictably and avoid fire; there's plenty of times when you will get flanked or outgunned and need to get away. I suggest playing rounds where you only try to knife people, getting in close like that means you have to dodge well.
3) Train your aim - Get in the habit of pulling the trigger for no more than 1 to 1.5 seconds, or use the M14 or M16 to restrict yourself to single bursts. With good control of your firing you can use just about any weapon at long range. You have no idea how much whining you hear when you knock down a sniper with a pistol, it really feels good.
4) Learn weapon accuracies - Load up a local game and find yourself a sign or barrel or what not that you can target from a long range. Practice hitting it while aimed, from the hip, crouching, and moving. The goal is to learn the firing pattern of the weapon, and how well it handles during movement and from the hip.
5) Learn to hear - Admittedly COD4 doesn't have the best of sound engines, compared to a game like FEAR where I could play with my monitor off and get an even kd ratio on pubs, COD4 I can only give rough direction for sounds. But it's still there. A good sound card and pair of headphones are important. After that, try just closing your eyes during a game, and listening, try to follow the sounds and every so often open your eyes to see how well you're doing. You'll look like a complete retard doing it, but eventually you'll be able to hear people coming up from behind and avoid getting jumped.
As for mouse binds - I use wheel click for reload, side 1 for ironsight toggle, side 2 for scoreboard, and the wheel lean for view lean. I use WASD movement, and bind sprint to E, grenade to Q, and swap weapon to 2.