Using the smaller, secondary, border crossings, avoiding the main I-87 Champlain crossing will greatly enhance the pleasure of your visit. This website shows current wait times for the Champlain (I-87) crossing coming into the US; scroll down to find Champlain. Also, avoid going on major holiday weekends or go early or late rather than at prime times.
Look at this map for easy directions to either of these two alternative crossings. The short detours to both of them are, in my opinion, preferable to waiting at the I-87 crossing. (To see the crossing at Hemmingford you need to move the map by clicking on it and dragging toward the right.)
The Border Crossing We Use (St. Bernard de Lacolle)
Lacolle is east of I-87 but not as far as Rouses Point. Route 276 in New York turns into 221 in Quebec. We have never waited in a line going into Canada here. Coming into the US we usually don't wait but there can be 30-45 minute lines at certain times on nice summer weekends and holidays. This is about a 10 minute detour off the main route.
The Hemmingford/Moors Crossing
Hemmingford is west of I-87 and is a bit longer drive but friends use it and say they have never, ever, even once, waited in a line in either direction. Route 22 in New York turns into Route 219 in Quebec. I used it recently and didn't wait. Check this map carefully before going as it is a little more complicated than Lacolle. If you get it right this is about a 15 minute detour off the main route.
Bus Service
There is also bus service between Montreal and Plattsburgh that is inexpensive and far far faster than using the train from Westport.
Limo Service
You can hire a limo service to take you to Montreal (or anywhere) and for a few people, it isn't all that expensive.
Charter a Plane from Lake Placid
To get to/from Montreal (or anywhere within about 350 miles) chartering a plane for a family or small group can be a reasonable option.
Traffic and Getting Around in Montreal
For information about traffic around Montreal check out this site. The bridges over the St Lawrence can be slow during rush hour times. During the summer, the city loves to put on street fairs which are great fun but bad for inner city traffic.
The Montreal subway is a great alternative to using your car in the city and in winter it is by far the most pleasent way to get around; you may be surprised to find that Montreal has a vast underground city. Traffic around the Montreal Airport is also a bit complicated and gets quite slow during rush hours.
Finally, Montreal has an excellent public bike system called Bixi and I highly recommend it
Wait...