In last month's Privacy Watch, I wrote about the best ways to clear data from an old PC's hard drive before you sell or donate it. But trading in a cell phone can pose an even greater privacy threat: People store PINs, passwords, and other sensitive information on them, and are likely to trade them in more frequently than their PCs. Also, wiping data off a cell phone can be extremely difficult.

If your cell phone stores contacts and other information on a removable SIM card, start by taking the card out. The SIM card doesn't necessarily store all the data on your phone, though. It may store only your phone book, while call logs, photos, memos, and other information might reside in the phone's internal memory.

To get rid of everything, you may need to employ multiple reset commands--and those commands aren't always easy to find in a modern cell phone's complex menus. One Samsung phone I looked at requires you to enter ten different commands to delete all data, including text messages, phone numbers, call timers, and logs. But remember, if you want to keep the numbers stored in your SIM card, by all means remove it before you delete anything!

The folks at ReCellular--a cell phone recycling service--have a great solution: The Cell Phone Data Eraser page lets you choose the brand and model number of your cell phone, and then displays the precise commands you need to delete every piece of data from it. (If you don't know your phone's model number, try checking underneath the battery.) If you can't find the instructions on that Web site, you'll have to find your manual. What do you do if you've lost that page-turner? Fortunately, most cell service providers offer downloadable copies of the instruction manuals for the phones they sell.

If you think you can circumvent the privacy threat by sending your phone back to your service provider, you could be mistaken. According to one report, a Cingular customer who received a refurbished phone as a replacement for one that malfunctioned found the new phone was filled with the previous owner's private data, including account numbers, user names, and passwords. (Read the full story.)

Once you've taken the steps that are supposed to wipe all traces of data from your phone, double-check to make sure your address book, call logs, and other data stores really are empty. When you're sure everything is gone, you can donate your old phone with peace of mind. Click Dialed In offers a list of organizations and companies that accept phone donations.

Andrew Brandt is a senior associate editor for PC World; you can e-mail him at privacywatch@pcworld.com.