When will your smartphone carrier throttle your data?Looking to defray the cost of the new iPhone 4S by getting some money for your old iPhone, Android phone, or BlackBerry? Well, you could try selling it on eBay or Craigslist, but more and more people are picking an easier route: companies and stores that exchange your old electronics for cash or credit.

Going the trade-in way means avoiding the hassle of finding a buyer for your old smartphone—but it also involves taking a leap of faith, given that you’ll have to wait days or even weeks after sending off your handset before you see a dime.

Don’t pack up your iPhone or Android device and send it off to a bunch of strangers without taking a gander at these five tips, starting with…

1. Shop around

Don’t just go with the first trade-in service you find on Google. After checking out just a few online trade-in sites, I found offers for a 16GB iPhone 4 in good condition ranging from about $150 to more than $260. Doing just a little extra legwork can really pay off.

2. Don’t forget the charger

The trade-in value for your phone will go up a notch if you include its original power cord—not a lot, mind you, but every little bit counts.

3. Wipe your phone before you hand it over

Some trade-in services promise that they’ll erase all the personal data on your phone as soon as they receive it, but don’t take their word for it. Make sure to wipe your phone’s memory completely before you drop it in the mail or hand it over to a clerk; here’s how to do it.

4. Read the fine print

So, you sent your phone in to a trade-in center, they looked it over, and they don’t agree with your judgement that it’s in “good” condition—and indeed, they may even decide that your old phone is worthless. What happens next?

Well, some trade-in companies will give you the chance to back out of the deal and ask for your phone back. Others, however, have more of a “sorry, you’re out of luck” policy—meaning that once you’ve sent them your phone, you’ll never get it back, even if they decide its value is zilch.

So, how can you make sure you’re dealing with one of the good guys? Simple: read the fine print, and make sure the trade-in company you’re considering will return your phone if you can’t agree on a price.

5. Go to the trade-in counter in person

If you’re really paranoid about falling victim to a disreputable trade-in company (and based on the number of horror stories I’ve heard, you’re probably not being paranoid), you might want to trade in your phone at an actual store rather than dropping it in the mail. That way, you can get an instant quote from a real person—and if you don’t like the quote, just take your phone back and leave.

Have more questions about phone trade-in services? Post ’em in the comments below.