I'm not super experienced with halter and showmanship, but I can try to help a little. First of all, what level will you be competing at? Yes, you can use human shampoo on your horse, but the cheapest I've found is Mane and Tail. It comes in a large bottle and is meant for horses:) If you are wanting to show in anything super competitive, you may want to look into a fake tail. If not, don't bother because they are expensive. I'm not sure what the more advanced horsemen use, but for small shows I use baby oil on my horses nose and eyes. Make sure you have some kind of product to oil their skin on their face. Also get some hoof paint. I paint mine black, but on my mare with all black hooves I just use clear polish. Trim the bridle path one ear length. You may already know all of this, just covering the bases. I wouldn't say a slinky is an absolute must, as long as you don't mind taking care of a messy mane before the show. If you're horse is light colored I would say it's much more important to have one than a horse with dark coloration (they all lay in mud/manure sometime so stains are just going to happen). I taught my mare to pivot by giving her the cue I want her to respond to with the lead line and coupling that with pressure just behind her shoulder. At first I really had to push at her and after a while she started to understand and I no longer had to add any pressure on her body, only with the lead line. Make sure she is squared before you pivot? She may be falling out of it if she doesn't get her hindquarters under her some. Again, probably best if someone more experienced with halter help you out, but that's just what I know of. Depending on what level you are showing, a button down shirt may be okay, but a show/rail shirt is really preferred and looks more professional. I show in halter twice a year so I bought one for ten dollars on ebay
. Also check craigslist, there are plenty of people that have grown out of their shirts or showing altogether. Keep an eye out, you can find something:) If you're horse has a thick tail, try using cowboy magic. It works wonders, however it is expensive, so maybe ask for it as a birthday present? Remember to trim the nose, ears, beard (if your horse has one xD), and fetlock. I hope I helped some, sorry if I just gave you a bunch of information you already know.