We don't tie our horses in the trailer for long (12+ hours) trips. We put the hay on the floor in front of them, just open a bale and spread it out. The horses can then lower their heads the entire trip and we've never had any problems with respiratory things. We have a 4 horse slant, we'd tie them if it was an open stock trailer without dividers.

We really don't do anything special or bring a lot of special stuff. We always have a first aid kit with just regular items in it, and always have a couple of extra rope halters and leads. We do just water them at gas stations but they generally are uninterested in drinking until 8+ hours. We also drive straight through and don't unload the horses or stop except for gas and food for us. Those stops give the horses plenty of time to rest.

We're going to Texas (from Wisconsin) in 2011 and then will camp overnight at least once. We use portable electric fencing so the horses don't have to be tied and can move around/lay down if they'd like.