All great suggestions. Allow me to add my experience.
I trailered my B5 about 200 miles from the dealer last Oct. I lashed the ratchet straps to the handlebars, as many of the previous posters had recommended, and tightened the ratchet until I had fork compression. Then I made sure both wheels were secured and proceeded on.
The problem was that the dealer didn't do such a good job of tightening the handlebars to the nacelle. After about 100 miles the handlebars were rotated downward, exerting excessive stretch on the clutch and brake cables. This probably did some minor stripping of the handlebars, but more importantly it left the ties loose, allowing the B5 to bounce along for who knows how many miles. Thankfully my paranoia coaxed me into stopping "just to make sure" everything was alright. The visions of my brand new bike, in pieces all over I-10, almost came true.
I now secure my B5 by the frame. There are two bars that run parallel to the seat, one on each side. I'll take the straps, run them under the seat to the opposite side, and the lash them around that bar. This cross-ties the B5 and, with the wheels secured to the trailer, prevents any motion at all. You could lash them over the seat, but this puts pressure on the cushion and the leather, which is no bueno.
Experience is always the best teacher. That's why this forum is so great- we learn from each others mista.. er, wisdom.