Match the sideseams of the pants and iron the creases - if you want to dedicate a pair to competition you can actually have them sewn in place at the very edge. If the pants are wool, your should press them through a scrap of cotton-like fabric, otherwise the wool will become very shiny from having it's nap crushed.
You MUST wear a tie, however you have your choice between a traditional long tie, or a bow tie. For competition, a bow tie should be black, though you have more flexibility in color choice with a long tie (for example, you could match your partner's dress). Clip on bow ties are fine, though real ones add an extra touch - ask one of the advanced leaders to show you how to tie one (it really is like tying your shoes, but the similarity isn't apparent until you've done it a few times). If you choose a long tie, you need to safety pin it to your shirt or wear a vest or cardigan over it - and tie it so it doesn't extend below the vest.
If you wear a long tie, wear a normal button-down shirt in white or possibly a color that suits you well - I've seen blue work quite well. You may want slightly longer sleeves than usual, as the cuffs may pull up in dance position. If you wear a black bow tie, you might consider wearing a pleated or pique weave tuxedo shirt though this is not necessary. Many would argue that a black bow tie should be worn with a normal turn-down collar that covers the band - the wing collar is for the white tie worn with a full dress tailcoat, but this rule is often ignored today.
While most shirts sold today are wrinkle-resistant polyester cotton blends, a 100% cotton shirt will feel much better in the heat of competition, at the small cost of a little ironing.
While you should not wear a jacket or tuxedo not specifically made for dancing, you do have the option of wearing a vest (waistcoat) to obtain a somewhat sharper look than a shirt on its own. Suit vests are okay and quite common, however anything with an adjustable or lining-fabric back is technically meant to be worn under a jacket, not with the back on view. You can get a washable black "waiter vest" for around $30 at Keezer's (140 River Street, 617-547-2455) designed to be worn on its owon. If you wear a vest, you probably won't need a belt. As for colors, I've seen blue and gold vests work in smooth, but it's not something to try without a lot of thought - black is a lot safer for starters. Cary extra safetypins for emergencies (yourself or your partner) clipped to the vest lining.
Another traditional option is to wear a black or perhaps gray cardigan sweater or sweater vest over your shirt. Some people really like this, though I personally find it too hot for an athletic activity like dancing - in fact I usually take off my vest or tailcoat between rounds.
The team has a few cardigans which can be borrowed, some veteran leaders may also be willing to loan out vests.
Black, unless you enjoy looking like Dilbert.
Whatever style you choose to have it cut in, try to get it at its neatest for competition. Anything longer than a crew cut will require hairspray or gel - do it yourself, or expect be to attacked by a spray bottle-wielding Elizabeth at the competition.
Simply put on a black top of some sort - something intended for clubbing, a simple black T-shirt, muscle shirt, or whatever your partner thinks you look good in.
If your costume isn't too complicated and the comp isn't too far away, it might be simplest to wear most of it there. I usually try to wear something similar which can serve as a sort of backup costume - in one case traffic delays meant all I could do was change my shirt while a friend pinned my number to a vest. If you do plan on changing, folding garment bags are the best way to transport your competition outfit. If your shirt requires studs or cufflinks, pick up an extra set.
While it's not strictly your business what your partner wears, it is in your interest to encourage her to wear her intended costume to a dress rehearsal. Many commercially available skirts and dresses are too restrictive for closed hold standard and smooth dancing - she may feel like she has room to step, but to move together as a couple you need room not only for her legs, but also for yours. If there are going to be issues, you want to discover them while there is still time to find an alternative costume - bouncing off your partner's skirt on each inside partner step is not a fun way to dance a competition.
Chris is willing to help with simple alterations - sewing pockets or cuffs closed, adjusting hems, sewn in creases, etc and usually travels to comps with an assortment of emergency repair items.