Comply with dress etiquette while keeping comfortable
The only thing I hate about surprises is not knowing what to wear. That's why starting out in competitions, I always had to ask my competition friends what I should pack or show up in. The same goes for the dog.
Aside from the proper collar or harness to use for each competition, you'll want to have a separate one for each sport. That way, your dog will know what they're meant to be doing! As soon as I take out Kooper's scent work harness, she gets very excited because she knows what time it is.
So here's what we would wear to each...
Conformation
Human: Although there is no dress code for conformation (despite what it looks like at Westminster), you want to wear something that helps your dog stand out without upstaging them. Go for colours that are the opposite of your dog. If your dog is black, don't wear black. As for the shoes, you'll want to wear something nice but comfortable enough to be able to walk/run around the ring.
Dog: They need a very thin slip collar and show lead. The aim is to have the collar high up and invisible. It should be easy to slip on and off. There are also some beautiful show leads so feel free to splurge on this. But again, it should not distract from the dog.
Scent Work
Human: Wear something comfortable and easy to move in so you're not distracting your dog. There is absolutely no dress code so feel free to wear trainers. Just don't forget your treat pouch (and have it open before you go in) so you can easily and quickly reward your dog.
Dog: You want a comfortable harness that's super easy to move in. Scent work is a pull sport so the harness should have a back clip. Though your dog should never have to pull you - you should be following behind. As for the leash, you want a 10-12 ft leash so you're not guiding your dog. They should be telling you where the scent is. Occasionally you may have to guide them but you're never pulling them.
Barn Hunt
Human: This depends on the time of the year. Keep in mind that you'll be outdoors for most of it so wear something warm if it's cold out or cool if it's hot. You can also watch other dogs for this sport so you'll want to hang around. I like to also have my ankles covered. You are in a barn after all and there could be mice roaming around which I'm terrified of. What you MUST remember though is that you can't have any cell phones, watches or treats in the ring. So leave all of that in the car.
Dog: Your dog will be running naked so you ideally want a slip leash or martingale collar to easily be able to take it off as soon as you're in the ring.
In general, other than for making sure you comply with the rules (such as no time trackers in barn hunt), it's important to be comfortable without distracting your dog.
Stay tuned for agility and rally/obedience attire.