As information sharing around residency opens up, border officials now have access to specific passport information not previously available to them. This gives them the ability, and authority, to enforce residency regulations.
The final phase of the Entry/Exit Initiative which came into effect last year (2014), tracks each day that a Canadian spends in the United States. The Department of Homeland Security retains this information. If you cross the border frequently, or for prolonged periods of time, you must be cautious of the 120 day threshold.
Penalties for being in the U.S. in excess of this allotted time frame are substantial. You may not realize that even a quick trip across the border for just a few hours could put you in jeopardy if you are close to exceeding your residency threshold.
Keeping close record of your visits on an IRS form 8840 is wise. Keep one with you in the vehicle, and record your stay whenever you cross the border if you are in the habit of spending a substantial amount of time to the sunny south of our border.