Add a new line to you account.
But will that actually increase my monthly bill? Yes, but it will only cost you $10 more than the existing plan. Rather than open a new account for an individual line that will cost you at least $40 a month, you can save $30 or more by adding an addtional line to your existing account. Many carriers allow you add up to 5 lines in a family plan account. So you savings can add up to $120 for addtional 4 lines.

Cancel features you don't use.
Do you do emails, web browsing, text messaging on your phone. If not cancel these features. Unlimited data typically cost $20 or more a month, text messaging cost at least $5, some email feature like the Blackberry email service can even cost more at $45 a month. These features and be added or removed without changing your exsiting contract. So cancel those features that you don't use often. This can decrease your monthly bill significantly especially you have multiple lines.

Monitor you minute usage.
You can easily check your currently minute usage on your phone or via auomated customer service. Many people sign up for plans that offer more minutes than they needed, fearing that once they exceed included minutes there will be huge overage charges. It's always easier to upgrade to a more expensive plans than downgrading to lower cost plan. So monitor you minute and feature usage every week, and try to cut down the usage if you think you will exceed the included minutes in the month. If you consistently exceed your included monthly minutes, simply upgrade to a plan with more minutes.

Choose carriers with rollover minutes or flexible minute plans.
Some carriers now allow you to use unused minutes from previous month. That way during those months with higer usage, you can still talk more without accruing any overage charges.