If you have teens, it’s a good bet they are asking for a smartphone if they don't already have one. And as the holidays get closer, that asking will get more and more fevered. It’s certainly easy enough to walk into a store and walk away with a state-of-the-art smartphone without a huge outlay of cash. But that’s because the real cost with a smartphones is the service plan. If your teen destroys that phone before his line is eligible for an upgrade, the replacement will require a much heftier outlay of your hard-earned cabbage. So it's a good idea to do a bit more thinking than that. Here are a few things to think about before you go shopping.

Go Waterproof

Water is--finally!--no longer Kryptonite for mobile phones. So if your teen is likely to drop it in the pool or washer. Or if she is prone to falling out of boats or walking into the shower with it in her hand, look for a waterproof model. It just might survive a bit longer. The Kyocera Hydro from Boost Mobile is a waterproof Android phone that costs only $79 on a $55-a-month unlimited plan. I tested its waterproof abilities by tossing it in a pool while filming video. (That's a photo of it in the pool.) It was fun. And it got some great footage from underwater. Here is the video:

Pay for the phone, Save on the bill You end up paying for those “free” phones by signing a two-year contract with your cell provider. And if your teens are like mine, no one uses the talk minutes you are locked into paying for every month. It doesn't have to be like that. Check out Ting.com. You pay up front for the phone. But then you pay only for the calls, texts, and data you actually use. Run the Ting savings calculator to see how your costs would compare to your current plan. A 100 minute-plan is only $3. But if you don’t use those 100 minutes, you'll get a refund. I tried it for a month –  it wasn't my main phone but I used it – and spent very little money. A smart phone can use Wi-Fi when a network is handy. So this pay-as-you-go plan can save you a bundle if your teen only needs cellular data occasionally. At the moment, you have to buy the phone from Ting (prices vary) but soon you will be able to bring your own device into the plan.

Bring Your Own Device

You can pick up a used smart phone at Glyde.com or Amazon.com for much less than a new one. Then all you need is the plan. If you bring your own phone to the plan, you don’t sign a contract and that means you can hold out a fancy new phone for a reward for awesome grades or turn off the teen’s plan if things go badly. Adding a teen to your own plan is cheap, usually less than $10 a month, plus data.

Data Sharing

Wireless providers understand that you want your cellular data – probably not in huge amounts – available to more than one device when you have a couple of teens in the house. So they have started rolling out plans that let you share one data plan with the entire family and all your devices. They are a bit complicated to understand. But if you do a little math, you might save some money. Check out the online calculators at ATT or Verizon to see if this will help your bill.

Turn it into a lesson–and a clean house

Set your teen down at Gazelle or Glyde and show her how to sell her old gaming devices, video games, music player, movies, and cell phones to earn enough cash to buy her own smart phone. Then make your contribution a no-commitment plan. The folks at Gazelle tell me that an iPod Touch can bring in $100. And even a broken iPhone 4s can net up to $100.  Considering you can pick up a refurbished Android phone at Glyde for less than $100, this could be a profitable way to clean the house.

Buy the Underdog

Windows Phone 8 launched recently. And it's a great smart phone option for teens. It has fantastic study tools (Microsoft Office) and games (Xbox built in.) But since the operating system is much less popular at the moment than the iPhone or Android phones, you can get great deals on new phones. These phones have beautiful screens, fantastic cameras, and the Windows interface is very personal and keeps teens in touch with you and their friends at a glance. My kids love it.

Christina Tynan-Wood writes the Family Tech column for Family Circle, and is the author of “How to Be a Geek Goddess.” You can find her at GeekGirlfriends.com. Follow her on Twitter, @xtinatynanwood.