google reentry permit

you have to apply before you leave the US. It is valid for 2 years and cannot be extended.

http://www.murthy.com/reentry.html

Be aware:

A person who spends most of her/his time abroad and returns only for brief annual visits runs the risk of losing the status. Upon providing valid legal reasons, however, one may be able to obtain a re-entry permit valid for a maximum period of 2 years.