There are several agreements that help you out with extending your patent rights abroad. The most important one is called the Paris Convention for the Protection of Industrial Property. This agreement provides that an applicant from one contracting State shall be able to use its first filing date as the effective filing date in another contracting State, provided that the applicant files another application within 12 months from the first filing date. Let me give you an example: If you file a first patent application for your invention on October 1, 2013 then you have one full year until October 1, 2014 to extend your patent rights abroad and the filing date in all those foreign countries will be October 1, 2013.
Another important agreement is the Patent Cooperation Treaty (PCT). Once a PCT application is filed, the applicant has 30/31 months for extending the patent rights abroad. That is, this treaty provides 18 more months for the applicants than the Paris Convention to file patent applications in foreign countries.
You will find more detailed information about the Paris Convention and the PCT in this FAQ section.