Multiple SMS Threads
Have an entry in 'Contacts' for each one of your friends (Contact -> Add new contact, then enter their number and details) and then enter SMS chat with them any time by entering into their respective 'Contacts' profile and clicking on the 'Send Message' icon to begin SMS chat.

Or just use an Chatting app like WhatsApp which has native support for all modern mobile OS's (Android, Blackberry, iOS..) so you could avoid retyping the same number again and again.

Or just stop mistyping those area codes.