McMaster - Carr sells a wide variety of gas struts.  Kim Brennan mentioned
that he installed some on his westy just for raising the top.  I suspect he
could post some pix of that clever installation on the yahoo vanagon group
to give you an idea of how to do it.

Kinda works like this.  You will buy two sets of struts, one for just the
top and one for the top plus whatever you have currently mounted up there
(cargo box, spare moose, that extra vaganza).

The struts help you lift the top.

Or you can just get several varying lengths of the ubiquitous PVC irrigation
pipe add a rubber crutch tip to each end and you have a prop that will help
alleviate the 'clean and jerk' move to raise the top.  Raise it a ways,
insert the first pipe, take a break, raise it the next step, insert the
second pipe...pipe is placed between the top and the ledge at the front of
the roof where the latch is.  Rubber feet keep it from slipping.

Of course you should carefully lubricate the hinges both at the rear
(they're plastic bushings) and at the front so rotation is as friction-free
as possible.