jumptoit's answer probably covers it, but I'd just add that you have to remember they use their heads for balance so they need to be able to move them around when travelling - a pet hate of mine is seeing horses tied super-short in lorries so they can't move their heads at all
I use the entire trailer for mine, with no partition, and thus she is cross tied for security. So, they are tight enough to prevent her turning around properly, but loose enough to move her head- can definitely turn her head to face backward as she watched me as I walked back to trailer yesterday.
I use a short bungee type rope in the lorry. Its short enough to stop her putting her head under the partition but has enough stretch in it that she doesn't feel restricted if she has to move for balance when we are on the move. It also has a safety clip on one end so it releases in an emergency.
When I had a trailer (with partition removed) I used to cross tie with ordinary ropes but kept the ropes long so she could move but not turn round.
I tie Adrian up as per RobinHood, but I didn't tie Catembi up at all. Can't remember why. He travelled a lot better (herringbone in a lorry by himself) if I left him untied.
Tie pretty much as everyone else, ordinary ropes short enough to stop them getting heads under the partition but enough rope for them to shift about.
When hanging about between classes though we leave the fell untied completely as she's a sweetheart so jsut stands. She also drinks tons at shows so it means she can help herself to water from her bucket if wanted. The tb is evil though and eats little ponies for her breakfast, so when at shows she gets tied short so she leaves the fell alone.
i cross tie so they can put their heads down and move from side to side but cannot get into any silly positions, turn around or get under partitions etc
In the trailer, mine are tied with trailer ties not too short - They can stretch their heads a bit and reach their haynet easily but not long enough for them to touch the floor!