Living in Canada, I trailer my FLHX to the Southwestern USA 3 or 4 times a year (winter) in order to ride.
My enclosed bike trailer has a wheel chalk which is important for stability. I simply use one set of 2" ratchet SoftTye tie downs
PowerTye American Made Tie-Downs,Motorcycle Tie downs,Big Daddy, Soft tie,Ratchet Tie Downs from the floor hook to the crash bars (to keep the bike pulled forward and balanced), and one set of tie downs from the floor hook to the handlebars (grips) to help keep the bike balanced.
A third set of smaller tie downs connected from the rear footpegs or saddlebag loops to the edge of the trailer floor. This prevents the bike from moving from side to side or favoring the crown of the road.
The key is to not put too much (if any) downward pressure on the front suspension, this can be done without any noticable compression on the front forks or major pressure on the handlebars. You just want to keep it from moving (
as rjvice says in his post) It may take a few times to get it just right but once you get it down, you can securely load the bike in a matter of minutes, and check it at gas stops.
One last thing, if I am going to leave the bike in the trailer over night at a hotel for example, I will relax the pressure on the tie downs to release any minor pressure that may be on the fork seals.
Good Luck