I did my first competition at the Pacific Dancesport Competition in 2000. I have notes from what I learned then.
This new information is from my Dallas, TX competition October, 2016.
You might want to try putting some sparkles in your hair. In addition to combs with crystals, they now sell hair pieces with crystals already attached. Another item I saw, was hair "necklaces". They look like a long necklace make of stones, with a larger stone in the center - like a pendant. But the back of the necklace has no clasp. The "chain" portion is also stones. You glue the center stone to the middle of your forehead, then wrap the chain portion around your hair. Its all held in place with bobby pins. They also had designs in stones that could be pined to the hair. They were actually pretty nice looking.
If your makeup looks good close up, you probably need to change it up. If you are doing your own makeup, make sure you know what you're doing. Its a nice idea to learn how to put it on yourself, though a number of women had their makeup done by one of the services. I did and I admit it was worth it. Some of the people that do makeup can also do hair. For reference, my makeup took about an hour to do - start to finish. I understand they began doing makeup early - like 4AM early. I had a 6:30 makeup appointment though I had an 8AM dance start. So factor for that kind of time.
Just for reference, the ladies I went to used MAC, Urban Decay, and Too Faced. They also too huge suitcase size makeup cases filled with makeup. Those were the brands I saw, but I know they had other brands in their cases too. And a myriad of brushes and eye lashes. Some services offered to just put on lashes for you if you needed that. I think lashes only cost about $20 maybe, and hair was about $85 not including anything you might buy from them. My makeup service cost $85, just to do makeup.
Take a small kit with things you might need. A small sewing kit is helpful but you may not have time to sew. Safety pins might come in handy, and are helpful to keep. Rather than just having a sewing kit, keep a few things like band aids, pain killer (aspirin or motrin), nail clippers, etc. in the kit. Think of it as an emergency kit, not just a sewing kit.
Take spare bobby pins, hair pins, pony tail holders, etc. Not all women wore their hair up, but most women did. Some had crystal pieces in their hair, some didn't. Make sure that gel or hair spray works before you go. I know the makeup ladies that did hair had a lot of hairspray, and used a lot on their customers. For those that danced all day, by evening I noticed some ladies' hair styles coming down. Try that new hairpiece before you go. You don't want to get there and realize its not going to work or it really doesn't match your hair color. Some of the hairpieces for sale had crystals already attached. They also sold a variety of hair pieces - some fit over your own hair, making a bun look much larger. They even came with a comb to help make it more secure, but they still pinned their hair pieces on. Nonetheless, try your ideas out at your lessons to see if they will withstand dancing.
You really do want to do a practice run in your outfit. Make sure it really does fit. If the dress needs a higher slit, get it fixed. If its too loose, take it to alterations. My new dress has a spare skirt underneath with a horse hair hem. It actually was a bit more difficult to dance in than I realized. I should have danced in it more before the competition. It affected how I danced.
Make sure you have all your accessories. Gentlemen - make sure you've got collars for your shirt and button covers if you need them. Don't forget your handkerchief. They had some great ties and handkerchiefs that had crystals already glued to them. Make sure everything fits.
Ladies - check that jewelry out in advance. If you need jewelry, you might be able to buy some at the compeition, but it would be helpful to have it in advance. Do you know how to fasten that necklace, even if it has a velcro closure? Don't forget your earrings. The vendors had a lot of earrings for sale. I even picked up some tassell earrings. One great thing was the variety of earrings colors available. They also had a large number of necklaces and bracelets available for sale. Its still best to have what you need in advance. But there were a lot of options available for sale, and of course you can find them online.
If you can, take your own water bottle with water. They'll probably have water on a side table somewhere in the ballroom. If its for sale and you don't have money with you, having your own water bottle may be a lifesaver.
A spare small towel is handy to have. If you're doing a lot of dancing, you can use it to mop up sweat. Tissues can leave lint - not a good look. And don't forget if someone has spilled anything, that's your shoes that are going to get wet soles. And you're the one who's competing. Towels can come in handy. Even a washcloth will do.
Take a sweater, jacket (or something) to keep yourself covered and warm when you're not dancing. It also gives you something to wear walking to and from the competition and your room. Does your studio or instructor sell jackets or kimonos with their logo? I saw quite a few of those on the dancers.
Ladies - if you do not have trucks attached to your dress, remember to bring some. You don't want to show your altogether to everyone. Spectators will be watching, and that's not the shot you want posted to Facebook or YouTube.
If you wear fishnets, check them. I didn't see many ladies wearing them that danced Smooth or Standard. I did see them on some of the Latin and Rhythm dancers.The other thing I saw on Latin and Rhythm dancers was glimmer on their tanned legs. It was like bronzer with shimmer on their legs. It looked nice. Do your shoes work with your outfit? Some of the Latin and Rhythm dancer's shoes had crystals glued on them. I'm guessing you need to make sure you're used to dancing in those shoes before competing in them.
Most people kept cellphones with them. It was easier to check than a watch, and you can take pictures with them. Another note about cellphones, you were not allowed to take video at the competition. You could take still photos. Hence a lot of cellphones taking pictures at the competition. They did make announcements about no videos.
Get some sleep the night before and don't forget to eat something or bring snacks if you are dancing all day. Ladies - if you drink too much can you get in and out of that outfit easily in a bathroom? A couple of small snacks could be helpful - nuts, an energy bar. But think before you get anything to eat - no chocolate, oily, runny or cumbly stuff that will fall and leave marks on your clothes.