What do you all do to help one not be nervous in the trailer?

Hay bag already doing... 

A buddy horse in the trailer works wonders sometimes too..........

Yup, I've had a couple horses that were super nervous unless they had a buddy. I found it's easier to haul a 2nd horse than try and get one to calm down.

I also give mine zesterra before I load them, I don't currently haul anything that gets nervous but they still get it. 

I've tried a buddy horse too and it does help some... I am going to keep hauling a buddy and see if that doesn't help her more and more :)  Thanks!!!

Like the others have said a buddy. Have you tried a calming supplement (given as needed)? I have seen good results on my anxious mare with THE calming powder and cookies. I do not use specifically for the trailer but I know it helps her because she doesn't pee on the trailer when I give it to her.

Many moons ago I had a yearling that had never been on a trailer until he came to my pasture.  Apparantly he got 'car sick', drooling the whole nine yards.

I started him by just loading him and letting him stand in the trailer and not go anywhere, then we went for short trips.  It only took 4-5 short trips for him to get over it.

Start feeding him every grain meal in the trailer, then let him out right away so he doesn't start to get nervous.