The degree to which your itinerary involves Canada lets out some of
the discount airlines. If ground transportation to/from Vancouver
works for you, the Seattle airport might open up more such options.
southwest.com, jetblue.com, usairways.com (a major that seems to have
absorbed America West, and which serves Vancouver), alaskaair.com, and
frontierairlines.com are among the sites worth a glance at their route
map for at least part of your trip, in addition to the major
international airlines you know about.

A possibility to keep in mind is the use of secondary airports near
your destination (possibly nearER your actual ground destination!),
like Islip, Long Island for NYC or perhaps Burbank, Long Beach, Orange
County, etc. for the LA area.

Some of these airlines have software apps (like Southwest's "Ding!")
or mailing lists for the bargain hunter. Two or three weeks in
advance will often do -- especially if it is feasible for you to come
a bit earlier (you're probably staying long enough into January) so as
to miss the peak of holiday travel, when (a) it's always a zoo and (b)
the airlines don't have to offer bargains.

You might want to use these resources to get an idea of the
possibilities and then consult a travel agent who's familiar with the
US.

Enjoy your trip,
--Joe