Originally Posted by
Albion
Have crossed border many times at Niagara which would be the nearest crossing to YYZ. As others have said, pick up your visa waiver form at the BA desk or lounge at LHR and fill it in so you are all ready at the border with passport and waiver form. Depending which crossing you use the immigration agent may ask you to pull over and in some cases you have to go into the office. Can take 20 minutes or so. You pay a fee I think $6 so either have a cc or cash ready.
I'd say you will have to go into the office every time - only exception is if you already have a stamped IS-94 as you've already entered the US on the same trip (and e.g. did a side trip to Canada).
Reason is that they now take fingerprints and picture at land crossings. This started doing this (much?) later at land crossings than at airports. And even at the most obscure crossing I always go through (Okanogan valley) they do it now...