hey Vikas,
when crossing borders via land you receive a paper I 94 form and you have to pay $6 for it.
for renewing I 94 based on passpoert expiry you dont need to stay out of us for long. you get an i 94 lets sa until dec 2013 because your passport exipry is dec 2013. if you have renewed your passport, then you go to the border mentioned in this post and tell the officer why you are re-entering and the officer will issue a new I 94.