Extension of stay in united states with a visitors visa

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I am on a multiple tourist visa(b1/b2) entry (valid till 2016) to united states. I came to US in july 2006 and acc to the I-94 form,i have to leave US in Jan 07(6 months stay at a time only!). I wish to stay in US for a couple of months more after Jan. I have heard about the extension form from the us immigration office, but i am scared to apply for it as i fear it might reduce my chances of getting an entry again at the port if i visit again. Can anyone pls tell me if its possible that i visit canada/mexico in Jan and come back to US, immdeiately, along with a new I 94 form!!!