What the best way to carry a Beta Evo 200 Trial Bike without damaging the fork seal and making sure the bike doesn't move in the trailer?

How far down do you safely take the forks down?

Pictures welcome if you can. Thanks a lot for your help.

In the days when I used a trailer I used two tie-down straps, one from each side of the handlebars near to the yoke, to a point near each trailer wheel. With the bike on the trailer and leaning to one side, attach the strap on the short side first then cinch the bike into vertical position using the strap on the other side. At this point about half the fork movement is taken up. There is no need to use a lot of force and there is no effect on the fork seals. The slots the wheels sit in prevent fore/aft movement. Use a loop of rope round the handlebars to avoid the metal hook on the strap bearing on the bars.

I agree,people overthink and overdo the whole strapping down thing. I use two webbing loops around the handlebars and two cam buckle tie down straps(no ratchets) In many years and thousands of miles towing a bike trailer i have never had one come loose or fall off. Sometimes they look a bit unstable but do not move far. You cannot use a fork saver like on a motocross bike because of the shorter travel and low mudguard but i don,t like them anyway as one flew out after bouncing on a bumpy road.

Securing a bike to a trailer can not possibly damage a fork seal (unless you attach a hook at that point and physically damage it).  The forks are designed to go through their full travel.  Pressure build up?  I run 80 psi and 180 psi respectively in the forks and rear unit on my mountainbike.  

I suppose if your forks spent hundreds of hours fully compressed it might eventually have some effect on the spring but I doubt it's worse than constant use.

Your keyword was "safely". However you secure your bike (frenchy3's description sounds pretty good to me) you must have some redundancy in the system.If one strap (any strap) fails, the bike must not fall off the trailer. This is something that's not necessary with a van, as the worst-case scenario is the bike falls over inside the van.

I used to carry my trials bike on a trailer . the easiest way I found , was to have 2 x loops in trailer floor (providing base is good) made with good strong webbing / strap material . determine what length they need to be with suspension about half  / three quarters compressed . wheel bike onto trailer , put 1 loop over footrest pull bike to opposite side - put 2nd loop over footrest .make sure straps are on pivot part of footrests , not on sharp serrated part .  just a couple of secondary straps to bars or frame . if in any doubt , where loops go over footrests , use a wear sleeve or pad to stop loops getting worn . hoe this makes sense .