Take a deep breath, smell that? It's wifi. It's usually in our homes, classrooms, and office. It's also in quite a few restaurants and businesses(except theaters for some reason) There's also a 99.9% that your smart phone supports wifi, which is great. Why is that great? Because wifi doesn't count against your data plan usage. Doesn't matter if you stream netflix all day long. So where ever you go, use wifi. For me, I have wifi at home, work and school. So outside of the 20 minutes I spend transit, I'm always in reach of wifi.
Built in Monitors. There's a small chance that you phone can monitor your data usuage. If you're using a jailbroken iphone or rooted android, there's a good chance that your phone can monitor your usage. The MiUI rom comes with the monitor app that's perfect for this.
From here, not only can it monitor how much traffic I use, but it can send alerts when I get close to going over. Best of all, it can automatically shutoff data transfer when I reach my quota.
If you dig in your settings, you may also find a toggle like this.
Don't buy Smart Limits. A lot of providers will sell you a service that prevents you from going over your data and text message limits. The entire purpose of avoiding overages is to save money; but you can't really save money if you're paying a monthly fee to save money. It just kind of defeats the purpose.
I understand that this may not work for everyone. They are a few places that don't offer wifi, but after watching your usage for a few months, you should know your limits.
What ever you do, don't try replacing DSL or Cable internet with tethering. Caps on tethering are ridiculous, the speed is incredibly slow, and it's a hassle. You can use the internet in your home to replace cableTV. You can use the internet in your home to replace your phone. You can not replace the internet in your home with tethering.

