How do I get a good K/D ratio or high killstreaks in Black Ops?

Alot of people say to play defensively, but when I do, I can never get kills. I'm just guessing what they mean is play so you're not running into the enemys spawn and get killed right away. But whenever I do I can never seem to get high killstreaks. I can easily go 20-2 in a game, but I can't go like 40-3. How do people like Seannaners and Hutch get such good k/d ratios with high kills. Basically, I just need to know a good technique to getting high killstreaks