Yakima makes a system called the 'Showboat'. Its a roller setup that attaches to your cross bars and is adjustable. You can move it out over the back edge of the roof and side to side. It works great for loading and unloading our boats. I can lift the front of the boat up and rest it on the roller and then climb on top and pull the boat up and onto the saddles. I've never had to do it myself, my wife always grabs the tail end and helps.
The roller system slides above the roof and out of the way or out over the back.
The more your boat weighs, the closer you want the back cross bar located to the back edge of the roof. As you can see in the first picture, the roller cantilevers out from the last cross bar. Our boats weigh 53 lbs each. I'm sure there are other options, but this works well.
Mike and Kris