Sometimes I need to add certain templates of text – السلام عليكم و رحمة الله و بركاته and و عليكم السلام و رحمة الله و بركاته in my case – in many of the e-mails that I receive or send, so that I don’t have to type that sentence over and over, so I did the following:

1. Open your account and go to Settings -> Labs
2. Search for “Canned Responses” and enable that feature, the page will reload
3. Compose a new mail, you’ll find a new button labeled “Canned Responses” from which you can “add”, “insert” or “delete” responses, don’t forget to delete your signature before saving a new canned response as it will consider it a part of the template, and accordingly will be added twice

Source: http://email.about.com/od/gmailtips/qt/et_templates.htm