I decorate at christmas. That's about it, since my barn doesn't have a loft and is metal sided with 2x6 kick board siding about head high. I do have stall plates and fancy bridle racks but that's about it. I have seen some very nice and very decorated ones as well though. Usually things of metal or horseshoes hold up the best in that sort of environment. I know of one barn that had stain glass windows installed in the tack room. Speed is right, don't put anything out there that you would mind getting dirty and dusty, because despite your best efforts, it WILL happen.
"Keep a leg on each side and your mind in the middle"