Segment 3D model in blender to get a separate OBJ goup

Procedure:
  
1) Laso select desired faces in edit mode - CTL+LMS drag (use vertex/edge/face selection as needed - buttons at the right end). First select roughly and unlselect (CTR+Shift+LMS drag) to refine the selection. Edge mode helps here.

2) Split by pressing 'p' (split P mesh/vertex/split)

3) Select the part in object mode (tab) and give object and data block name in Link and Materials window.

4) Select all and export as obj (Say export blender objects as OBJ groups using the buttons at the bottom)


Refs:

http://wiki.blender.org/index.php/Doc:Manual/Modeling/Objects/Groups_and_Parenting
http://wiki.blender.org/index.php/Doc:Manual/Modeling/Meshes/Selection