Generally, I only use straps on the factory racks on my Highlander for transport around town. Recently, my family and I took about a four-hour road trip from Virginia Beach to Montebello, VA (partial travel along the Blue Ridge Parkway - beautiful!). As most of this was 65+ MPH interstate, I obviously wanted to use supplementary lines on the bow and stern of my Snowshoe Explorer 14.
I haven't attached any fixtures to the breasthooks nor have I set up any other way to tie a line to my boat, so I came up with an approach that worked extremely well. (I've placed two picts into my "KPW - Snowshoe Explorer 14" photo album.) Basically, the line loops over the entire hull, with a secondary line (which only puts a bit of lateral stress on a gunwale gusset block) holding the main line in place.
Note that the main line is quite a bit more "robust" than what would be required. I've learned over the years that thicker rope is much less likely to set up an annoying oscillation at interstate speeds.
Others may have approaches they like better - I was very happy with the way my boat traveled, so I thought I'd share what I came up with.