Setting your own e-mail signature creates a personal feel to your e-mails. With newer Android devices shipping with some pretty ridiculous default e-mail signatures, you'll want to make sure you change yours.
This slideshow will show you how to change the Gmail signature on devices running Android 4.0, which is different than how it was done on devices running Gingerbread or earlier. If you're using a Gingerbread (or earlier) device, I'll try to point out the differences along the way, or you can look at this previous post for some specific walk-throughs.
After launching the Gmail app, tap on the menu icon.
For those using a Gingerbread device, tap on the menu button, select More, then Settings.
With Android 4.0 you're able to set your e-mail signature for each account you have setup. In the past, you are forced to have the same signature for all of your accounts. Pick the account you'd like to edit your signature for by tapping on the account name.
Older devices you can simply select Signature and then edit the text of your signature.
You can now enter your signature. Keep in mind that your signature can be multiple lines, and can include Web site URLs, e-mail addresses, or your phone number. If you really want to add a personal touch to it, include a short inspiring or amusing quote for your recipients.
If you want to change your signature for additional accounts, just tap on the back button until you get to the settings screen and then select another account. If you want to set the same signature for all of your accounts, type it into the first account, copy it, then paste it into each additional account's signature field.