Make sure your horse is immaculate. Clean everywhere to perfection, under the tail, 'delicate' areas, eyes, nose. Same goes with your outfit and tack. Everything's gotta be spotless, no grease clogging the stitching, no smudges or dirt, no stray horse hairs on your clothes. Clean the stirrups really well too, expecially the treads as they may well ask you to take your foot out of the stirrup and have a good look. Also, make sure your horse is turned out how best fits his breed. Plaited or unplaited, trimmed or not trimmed, check what is correct turnout for your horse. This includes tack, like colour of saddle pad and girth and style of bridle. Not everyone can afford a full set of show tack (me included! The only show-specific thing I've got is a saddle pad at the moment!), but the judges should take this into account (especially at local shows). Don't let your horse eat before you go in the ring (to avoid green froth) and wipe off your boots (including the sole as Your Horse said) once you are on board. I've done turnout once and personally found it too much stress! Especially with a traditional cob, they just don't look as well-turned out as a flashy plaited show pony!