This only works by phone. Be nice, but firm. Don’t be satisfied with their first answer – make them escalate you to the retention department. They’re often authorised to give much larger discounts because it’s cheaper for them to retain customers than to recruit new ones.
If you’re not grandfathered into a sweet rate on a major mobile carrier you can potentially save a ton of money by going pre-paid. I’ve been using www.usmobile.com which is $30/mo for 35gb of prioritized verizon or t-mobile data ($25/mo with multiple lines). If you don’t use much data you can do $10/mo for 2gb.
Great tip, thank you.