This is for iPhones still using older operating systems (before November 2012). There is a new video for separate signatures on multiple e-mail accounts. You can watch the iPad video, as the process is the same on the iPhone. Click here to go to the new video now.
When you receive your iPhone, all e-mails you send out say “Sent from my iPhone” by default. It is easy to copy and paste a new signature into the settings. You can use a similar signature from your e-mail program (i.e. Outlook) to make the e-mails you send from your iPhone look professional.