Some protips for Demolition:
First, don't try to take a bombsite solo. Everyone likes the idea of single handedly taking down thousands of enemy soldiers and blowing their shit up, but this isn't a movie and you aren't an action star. Chances are soon after planting the bomb, overwhelming enemy reinforcements will kill you and easily disarm your plant. Instead, wait for your own reinforcements before planting. Without planting you won't draw any attention to yourself and you should have a teammate within 30 seconds to plant that bomb. Once planted, secure the area and don't die!
Which leads me to tip #2. Use the tactical insertion! You can be your own reinforcements when it comes to keeping a bomb going, getting back in the action within seconds. Always have it planted nearby in a corner or some place not traveled through often. The funny thing about it is few people use the insertion and fewer still even recognize the blinking light for what it is. It isn't unusual for the enemy to run right past it and I'll spawn behind them and kill them. It even beeps!
Round 1 doesn't matter, so don't waste killstreaks on it. Save all those for round 2, which matters much more. With all the saved up killstreaks you can be better prepared to assault or defend the bombsites for a clean round 2 win, meaning either victory or a 2nd chance at victory. You don't necessarily need to save anything for round 3 unless you lost round 1 and you are pretty sure you will win round 2, then you might save something to help ensure round 3 early on. Only once has this been an issue for me, and it was round 2 on Nuketown. I had so many care packages and SAM packages saved up after round 1(30/9) that I couldn't use them all before round 2 ended. I even had a gunship I couldn't launch because the airspace was full
Killstreaks that are effective in demolition might be different than ones used in other game modes. Mortar team, for instance, is very effective. Care package is very helpful, especially if you have hardline pro, but napalm strike can be extremely useful in most demolition maps too. SAM sites are a must! Because you have static respawns, it is a lot less likely that they will be destroyed and choppers are a lot more of a problem in demolition because they are better placed for assault and defense. SAMs also shoot down care packages which can limit what your enemy can do to you, a huge plus.