Staying alive..

18 kills, 18 deaths isn't so bad. But wouldn't you prefer 18 kills and 4.. 5 deaths?
Dying is inevitable in Modern Warfare 2 but dying as little as possible will definitely help your team and will also allow you to build up your killstreaks for that elusive AC-130 gunship, chopper gunner or the nuke!

A good class to have (assuming your playing Team Deathmatch like majority of people are) would be to have an assualt rifle equipped with silencer (for small - medium maps) with Sleight of Hand as perk 1 to boost your reload speed for those waves of enemies or Marathon to provide unlimited spring (Marathon Pro), perk 2 would be handy to have either cold blooded or stopping power to take down foes quicker and perk 3 can be decided on your personal liking, I tend to go for Commando to allow a lunge knife kill from a distance.

Keep running especially on TDM, Hardcore it's alright to camp (campings for noobs though =P) due to the lack of a permanent UAV. Continuous movement around the map eliminates enemies sneaking around to take you down cos they know where you are camping. The silencer will allow you to be invisible whilst taking down enemies and the stopping power perk will allow for quick kills with minimal bullets. Do routes on a map, in other words, continuously run around the map in a pattern, this allows you to get familiar with a particular area knowing where enemies can and can't be and enemies, more often than not, come back for some payback. When turning corners always aim down sights (ADS), it's more accurate than hipfiring and allows you to take out foes quicker if surprised after turning a corner.

There's more to it to staying alive but that's some of the key points!
Below is a video of an amazing killstreak, enjoy ;)