From About.com:

Stick to your signature look

The need to sport a suit does not mean you have to change your whole look. There's an endless amount of ways to let your try style shine through your tux. Things to think about when considering your prom night outfit: jacket length, color, and, of course, what you choose to wear underneath (oxford, vest, tie).

Coordinate with your date

Before you even consider your tuxedo options, check with you’re date about the color and material of her dress and if she has any preferences on your tux style. A coordinating cummerbund or tie adds a very cool touch to your whole prom look; plus your date will be thrilled.

Don't be afraid of trendy

It isn't good to go completely trendy (powder blue polyester suits from the '70s do not make good memories). But try these small, trendy details:
• Smooth cufflinks or a funky lapel trim.
• Bow ties are the traditional choice, but regular neckties look sharp as well.
• A vest always looks good, but a cummerbund adds a classy touch.
• Take a cue from celebrity red-carpet style and don a monochromatic (all one color) look or add an unexpected splash of color.

Little details do count

You may not think that things like your hair, cologne, or accessories (Guys? Wearing accessories?) matter too much, but they really make or break your entire look.
Add polish with these details:
• Have your hair trimmed a week before prom and try styling it a few different ways to see what you like best.
• Lightly splash on a favorite cologne
• Accessories that add polish to consider: cufflinks, hats jewelry (watches, necklaces, etc.) and don't forget the shoes (make sure they are shined, with no scuffs on the heels)!

How To Tie your Tie:

1. Begin by lifting up the collar – “popping the collar,” if you will - of your shirt, then place the tie around the back of your neck. The wide end of your tie should hang down about twice as far as the skinny end.
2. A few inches below your neck, wrap the wide end of the tie around the skinny end twice.
3. Push the wide end through the back of the V-shape made by the almost-formed knot.
4. Tuck the wide end through the front loop of the knot, then tug down on both the skinny and wide ends until the knot is tight.
5. Holding the skinny end, slide the knot up to your neck.
6. Flip your collar back down and voila! You are now a tie-tying pro!

Tips:
1. If the skinny end hangs below the wide end, untie the tie and begin again, with the wide end hanging lower than it did the first time.
2. If, on the other hand, the wide end hangs too low, untie the tie and begin again, with the wide end hanging higher than it did the first time.