I need to sell my old iPhone 4S. What steps do I need to do to ensure it's completely cleared?

I've already activated my new iPhone; my old iPhone can still receive iMessages when on Wi-Fi.
I'd like to sell my old phone. Should I "unlock" it? How do I do that?

Just some general info/instructions on what I need to do would be great.

Ok to ensure that all of your personal info is cleared go to the settings icon then to general and if you go to the bottom of the general your going to see a reset button, press that and it will erase all of your data. For the seeling of you device you can go to Gazelle or even verizon or whatever carrier you have. If your not happy with the quote you get from these you can use craigslist.

Don't unlock it. Somebody else's problem. Three basic things:
1. You need to factory reset it. Go to settings, general, reset and then choose "Erase All Content and Settings."
2. Remove your entire iCloud account from it.
http://support.apple.com/.../kb/PH2702

http://support.apple.com/.../kb/HT5661
3. Take your sim card out of it.
Then sell it (stating which carrier it is on) as you wish.
Best luck.