1. Get very familiar with the maps. People tend to hide in certain places over and over again...why not toss a grenade in those spots before you even go in?
2. Move! Stationary targets are easier to hit. Plus, when you do stand still, it tends to be in one of those predictable spots mentioned above.
3. Do the unexpected. Some of your future victims haven't begun to think in three dimensions yet, so attack from above and/or below whenever possible.
4. Teamwork is often the key. Communicate constantly with teammates, and go into every scenario with a plan. Refine it after each try so that you're always improving.
There are lots of little things that you can pick up on as others do them to you. My best advice is to learn from those who pwned you. And have fun...it is just a game after all. Letting yourself get stressed and upset about the game will not help you play any better, so relax and enjoy it.
Hope this helps!