Step 1: The Expensive Parts
You can get a hitch and mount it yourself, or a Uhaul, Sears automotive department , or similar will sell you one and mount it for you.
The next part you will need is a "truck bed extender" I got mine from Harbor Freight.com (p# 39168-2VGA) for 30$, it now lists for $39.99.
The third part you will need for a minivan or car is a 2" square steel tube that fits into a 2" reciever hitch, I got mine as a left over from construction at work.
The last part you will need is another draw bar pin like the two that come with the bed extender. I got mine cheap at Home Depot, but harbor freight, jcwhitney, or any trailer hitch type place will have them.
This picture is from the Harbor Freight catalog, I used to haul my canoe upside down on the bed extender just like the boards are shown in this picture.
Step 2: Modifying the Bed Extender for Roof Top Use.
Step 3: Now for Some Height
Step 4: Setting in the Cross Bar
As you can see in the picture this allows me to load and unload the van without having to remove the 2" tube section, the tailgate comes up to high to open it with the canoe on the roof though.
Step 5: Now Comes the Hard(est) Part
You need to pick up one end of the canoe and put it on the cross bar.
I then tie the end of the canoe down using the rope that is attached to the inner "ears" on the cross bar to keep the canoe "locked" onto the cross bar at the correct length.
I then tied the canoe down with the rope that attaches to the outer "ears" on the ends of the cross bar to keep the canoe from sliding from side to side.
Step 6: The Rest Is Easy!
I make sure that the nose of the canoe falls just at the peak of the windshield, this keeps the drag from the canoe down and doesn't slow the van down noticably. I can do this because the bed extender is considered part of the frame of the vehicle, so the overhang only starts aft of the cross bar.
This is the point when you put in the upper lock pin to keep the cross bar and 2" tube secure to the pipe. Then I throw an endlless cargo strap over the front of the canoe and strap it to the roof of the van (open the doors first so you don't have to do the dukes of hazzard climb in the window trick) the buckle goes in the middle between the front seats which makes it easy to keep an eye on the strap tension. Make sure you twist the strap at least 3 times on each side of the canoe so it doesn't hum like a giant hornet on the roof of the car.
I tie the forward rope of the canoe over the top of the van, and over the carry thwarts in the canoe (to save the paint) and down to the trailer hitch, this rope is to stop the canoe from sliding forward. I tie the aft rope from the canoe to the reciever hitch also to keep the canoe from sliding backwards, so far this has worked for me for almost 2 years on very bumpy back roads and at highway speeds with no problems. I like this way of rigging the tie offs as there is no rope chaffing my hood or the plastic front bumper, and I don't have to lie on the ground to feed the rope through the grill holes in the bumper to find something solid to tie off to.
Yakamia and/or Thule both make similar products that swivel for kayaks and canoes (without the wide cross bar) they both rise straight up from the reciever hitch though so clearance for a tailgate will be a problem, and they are both expensive. This set up will also work for ladders and lumber (within reason), something the expensive ones won't do as well.