Pretty general tip, but so many times I see people think it's a race to get to the mission start, and the first person there ends up starting a massive firefight with all the AI and getting spanked. Just wait for your team. All go in together. Makes missions so much easier, having those extra guns. It's not a race.
Oh, and use your car as cover. Don't use a bike on missions where AI cars are following you. It's useless. They'll chew you up.