I used to play singles on MW2 SnD and they are pretty easy to do well at. I dont know if this still applies to MW3 but here's what I used to do-
Offense:
Make your way to the easiest bombsite to plant on - cautiosly - dont sprint unless you know your safe. WAIT to plant the bomb. Most people will try to defend BOTH bomsites when they're on defense. A lot of times I find that I win offense rounds by killing him rather than planting the bomb.
Defense: I would suggest defending one bomb site, switching off every round. If he chooses to go the other way, then he gets a free plant. I find that if I try to defend both I can never make it there to prevent a plant, thus he gets to plant every time. Obviosly dont stay too close or in the same spot or you will get stunned, the killed. If he goes your way I'll trust you can pick him off.
If he goes the other way and takes the free plant, dont rush it. You have plenty of time. Sometimes I even needed to tell myself that to stop myself from rushing in. The longer you wait, the more likely he is to show himself. Everyone doubts themselves so he'll probably end up checking the bomb site at least once, so be in a good position.
Don't throw stuns and nades at the bomb. Its a noob move. He's obviosly not going to be there, so all you're accomplishing is letting him know where you are.
Hope it helped