Analysing the CostsGiven that the majority of functions on a smart device utilise the internet, you can arguably separate your costs between data costs and typical phone costs.
Typical phone costs are those that come from texts and phone calls. These can be expensive if you use your phone frequently, but for most people this is fast becoming a minimal use of the device.
On the other hand, you have data costs. These are used when accessing the internet, e-mail and social media. As such, these tend to be more expensive.
Cutting CostsTo cut these costs, you simply need to address each area separately. For typical phone costs, you should consider your payment plans. If you only make occasional phone calls and texts, there's no point having a large contract where free sim cards will suffice.
As for data charges, this is a simple matter of learning where and when not to use such services. In short, when you're at home or near a Wi-Fi spot in public, you can use this as often as possible. This data isn't charged to the phone, whereas using the likes of 3G is. Wise choices in this area can drastically cut down your expenses.