If you are going to type a long text and you know you're going to go over the 160 limit, you could select pix message and just not insert a pic. This gives you 1000 characters.
Hope this helped.
The 160 character limit comes with the SMS (Short Message Service) 'standard'. The suggestion to use a pic message is the best solution.
Here is a good explanation of SMS & MMS. http://en.wikipedia.org/wiki/Multimedia_Messaging_Service