On the maps where you have to hit a vehicle with the Ion-rifle(?) I use my Heavy and hit it with my Sentry ability. Usually gets me a couple thousand BP in one go. Of course, getting your team to protect you, while you're trying to get that shot off is highly unlikely. But in those rare instances where I get it, yeah, easy $$$.
Kill Streaks, as mentioned before, can also work quite well, but they're not easy to get for us lower-skilled players.
On tight maps I also like to use Officer and buff large groups in tight corners. That can get you quite some BP.
Kills around the objective seem to help too.
Other than that, I'm not good enough to ever rack up a lot of BP.