I have done this before and the trick I used was to glue an old piece
of carpet to a piece of 1/4" plywood. The carpet will slide along on
the flooring not to be harmed while the 1/4" plywood will be used as
the surface to which a pry bar can rest upon.

The hardest part is getting it started. Once started, slide the
carpet-plywood jig along under the edge of the old sub-flooring, using
the pry bar to pry little areas at a time. In my case I was able to
free a couple feet section and then grab a hold to it with my hands and
lift up. Usually several feet came free at a time.