As reiterated in a recent New York Court of Appeals decision (Galetta v. Galetta), separation agreements and pre-nuptial agreements must be properly executed (in the same manner a deed is executed) and must be notarized. The failure to do both of these items will almost always result in the agreement being invalidated.
This is why there's a danger in getting these forms off the internet. Many such agreements do not conform to the individual state's requirements regarding such agreements. They are thus about as binding as if you draw up something on a cocktail napkin. Stick with a reliable source - have it draw up by (or at the very, very least reviewed by) an attorney experienced with matrimonial matters.