It's a multi stepped process: use different IPs for different clients, warm up the IPs to build good reputation, actively work with ISPs to whitelist the IPs, enroll in loopback programs with the ISPs, carefully monitor spam complaint and bounce rates, etc.
Sending a large amount of email to a well established list can actually help your reputation, since the ISPs see that you send large numbers of emails that don't get reported as span -- that makes the (hopefully) few that do not affect your reputation as much.