I have a key bound for this, so I waste no time typing while I'm moving out of the respawn point. Once you get two or more teammates to listen to you, BP2 is effectively yours. The T's, no matter how large in number, will not be able to plant the bomb if there are at least 3 CT's guarding it. Add a sniper into your group and you can pick off T's in the tunnel. If you wish to camp at the respawn, I recommend that you jump on top of the crate pile to the right of the main arch.
Most people never look up before it's too late.
Another good place to be is at the respawn itself. From that ledge, you can guard the dark tunnel and the main area with ease. Sniping on this map is a very effective tactic. If your team is on a losing streak, then buy a TMP every round and use the strategy I outlined at the beginning. The TMP is especially good for this because it has a fast rate of fire, you can easily get headshots, and it's silenced.