Ramp angle and dark to light.
The nearer to horizontal you can make the ramp, the easier they will travel along it.
They move more readily from dark to light.
We have a building with a raised floor, and it's dark inside. We set the trailer up backed up to the door, ramp onto the step so it's almost level. Hurdles and side rails to stop escapes - but to let the light flood into the trailer and onto the ramp. Then we open the door into the building and shoo the sheep in the direction of the trailer. 7 times out of 10, they just walk in, because it's lighter out there, and no scary climb or descent, and it's away from the bothersome 'shooshing' 2-legses.
Oh - and make the trailer and ramp smell of sheep. So don't use clean straw, use used straw.
